Order of attacks might matter.

War Wizard is attacked by 7 archers. She uses Arcane Deflecton on first attack which rolls high enough to hit, but low enough to be deflected. Only 3 archers are in range of Deflecting Shroud, after using it, two of them die.

Were those two archers the ones who missed their attacks? Who hit before Arcane Deflecton was used? Or were they supposed to attack after this attack was resolved? Depending on order, their attacks might be negated.

Abjuration Wizard concentrating on a spell and protected by the Arcane Ward is attacked by same archers. All hit. 6 attackers have rolled pitifully low for damage, for 2-3 damage each, and one lucky guy critically hit for 20 damage.

If low-damage attacks hit first and only whittle down Ward HP, and then heavy-hitting attack comes and breaks the Ward dealing the rest of damage to the Wizard, the Wizard only needs to roll a single concentration check. If heavy-hitting attack comes first, breaks the Ward, and is followed by 6 weak attacks, the Wizard must roll 7 concentration checks. Oh, and a friendly bard might want to use Cutting Words to reduce damage. If he reduces damage from chosen enemy to 0, then it is one less saving throw to make. So you must track how much damage is dealt by each creature.

Frame challenge: avoid lumping monsters' turns together.

I advise you to read this answer on the effect on balance of having multiple NPCs take their turn simultaneously or consecutively.

Let's compare two alternative timelines:

1. Lumping:
   • T0: All 3 NPCs launch a volley at the caster, the caster falls unconscious.
2. Separate:
   • T0: NPC1 launches a volley at the caster, the caster cries out in pain.
   • T1: The big bad fighter takes in turn, steps between the NPCs and the caster.
   • T2: NPC2 launches a volley at the big bad fighter, the big bad fighter grunts.
   • T3: The ranger shoots at NPC3, kills it.

When you have a fight, the more characters from one side can act in an uninterrupted chain of turns, the more powerful the "combos" they can pull off are, often resulting in one (or several) of the characters on the other side dropping out of the fight (unconscious or dead). This makes for very swingy encounters, see also Rocket Tag.

Proposal: alternate sides.

Remember that the round in D&D is a 6 seconds time frame during which all characters act (in the game world) more or less simultaneously. The turns, then, are a pure construct to allow orderly resolution of the combat.

Given the problems posed by having too many characters of one side acting right after one another, I would suggest that, as the DM, you ensure this doesn't happen -- fudging the dies if you have to -- and instead aim for a more or less "uniform" distribution of the turns across the sides.

Note that D&D 5.0 introduced Legendary Actions specifically to break up the contiguous string of PCs' turns when they fight a single boss.

My recommendation, therefore:

• Let the X PCs roll initiative.
• Group the NPCs in X groups of roughly equivalent CR, roll initiative for each group.
  • If the NPCs cannot be split, it's fine having a single NPC be 2 groups or 3 groups worth of CR... but then they count as 2 or 3 groups in the next phase.

Then, fudge the initiative so that no 3 PCs nor 3 NPC groups act immediately one after the other.

Note: if a NPC has a Legendary Action, then it's fine to have the one lump of more than 3 PCs as you'll be able to interrupt it with the Legendary Action as necessary.

Beware of metagaming

I also advise you to be careful about not metagaming. It may be the smart thing for all 3-4 archers to focus fire on the caster (or any other PC), but:

• Are you sure that they would recognize the caster?
• Are you sure that, even recognizing it, they would think about targetting the caster?
• Are you sure that, even thinking about it, they would be stoic enough to actually do it?

Trained veterans obeying a veteran captain? I would think they would. Bandits or folks from the local militia? I would expect the freshest recruits to fire on whatever the closest target is, if they do not turn heels and run or stand there quaking if fear.

I personally find it much more interesting when both PCs and NPCs role-play, even in combat.

I agree with NathanS's answer, but I have a frame challenge

What about a character's Reaction?

In the original question, no mention was made about handling Reactions. It has since been edited in, but my answer still stands as an important consideration

By making things one lump sum, you are removing the chance for a character to use their reaction.

Consider that after the first attack, the player throws up a Shield spell. Now all the attacks need to be measured against a better AC. Did you remember all your rolls and can adjust accordingly?

What about Hellish Rebuke to throw damage back at the attacker? Or the Protector style from a Fighter?

There are numerous ways that the fight can change based on a single Reaction but it's all washed away by lumping everything together.

1. Yes, your understanding of the RAW on this is correct

RAW, each attack by each creature is resolved in sequence:

Initiative

Initiative determines the order of turns during combat.

So at the moment when the player would be dropped to 0 HP, they would then fall unconscious and prone:

Falling Unconscious

If damage reduces you to 0 hit points and fails to kill you, you fall unconscious. This unconsciousness ends if you regain any hit points.

Unconscious

[...]
The creature drops whatever it's holding and falls prone.
[...]

This means that any subsequent ranged attacks would be made at disadvantage:

Prone

[...]
An attack roll against the creature has advantage if the attacker is within 5 feet of the creature. Otherwise, the attack roll has disadvantage.

Furthermore, any that did hit would indeed result in a failed death saving throw (but only the one failure, unless the ranged attacker managed to roll two natural 20s for a critical hit at disadvantage):

Damage at 0 Hit Points. If you take any damage while you have 0 hit points, you suffer a death saving throw failure. If the damage is from a critical hit, you suffer two failures instead. If the damage equals or exceeds your hit point maximum, you suffer instant death.

2. No, Multiattack does not change the above

If a creature has a Multiattack action (or a PC with Extra Attack, for that matter), it simply allows the creature to make multiple attacks with its action. Each attack must still be resolved in sequence, so this does not change the above section, where each attack must be resolved in sequence.

The following somewhat related Q&A discusses how multiple attacks via Multiattack are still resolved in sequence: Can a reaction interrupt multiattack?

3. On initiative ties, each creature would still have to take their turns in sequence, they cannot act simultaneously

This is implied by the last paragraph of the Initiative rules:

If a tie occurs, the DM decides the order among tied DM-controlled creatures, and the players decide the order among their tied characters. The DM can decide the order if the tie is between a monster and a player character. Optionally, the DM can have the tied characters and monsters each roll a d20 to determine the order, highest roll going first.

Where there is a tie, the rules give various options for determining the order, which heavily implies that there being a discrete order is mandatory. There's nothing about creatures being able to take turns simultaneously.

Effectively, your choice to have them all shoot at once is a houserule, although I understand that it does make sense from the perspective of speeding things up. RAW, they do not act simultaneously and must take their turns in sequence.

However, in your case, I imagine you still want to continue rolling their damage together to speed things up, and that's fine.

I remember playing under a few DMs who did exactly this; in cases where multiple attacks hit a PC, some DMs would rule that the damage would reduce them to 0, but was kind and ignored the death saves for that volley, so effectively the PC was filled with arrows and dropped, but not killed, and got to roll death saves fresh at the start of their turn.

Other DMs rule more harshly, where they claim that, because the arrows or bolts all hit simultaneously (even though, RAW, that wouldn't happen), they weren't actually prone when they hit, therefore the attack rolls stand and ignore the disadvantage. They also tallied up the death saves that would have happened, although I think there were few enough enemy archers in this scenario that it was easy to determine that they would only suffer one or two death saves, and I am yet to see a DM basically outright kill a PC with a volley.

Personally, if I were to use this houserule of having enemies attack simultaneously, I'd err on the side of the players and let the death save failures slide, as per my first example above. However, what you choose to do is entirely up to you, but it does seem from your question that you understand the situation regarding making use of this houserule.