Minecraft PE, help me make a command to give me a multishot crossbow?

NBT tags have not (yet?) been fully implemented into the pocket edition

The only implemented tags that I am aware of are "can_place_on" and "can_destroy", which I did not know until I read this, I thought that no tags were implemented yet.

Note: I have tested some other tags and I could not find any other tags that worked.

You could do this, but this would replace whatever item you currently have in your main hand. It may not work propperly if you quickly switch between items.

The right command block has this command

/replaceitem entity @p slot.weapon.mainhand 0 crossbow 1

The left command block has this command:

/enchant @p multishot 1

Note 2: The repeater is set to 2 ticks, because 1 tick leads to some really weird behaviour. You cannot set the level of the enchantment "multishot" any higher than 1 in the pocket edition

You can have a chest with an enchanted crossbow (or any other item) that you clone to the position of the player and then you imidiately destroy the chest by using /setblock in a chain command block.

Your setup may look like this:

The impulse command block clones the chest to the position of the player with this command:

/execute <target> ~ ~ ~ clone <chest coordinates> <chest coordinates> ~ ~ ~

And the chain command block will set the chest to air, while dropping the item(s) inside:

/execute <target> ~ ~ ~ setblock air 0 destroy

Note: Other players may be faster at picking up the item(s), they will simply drop to the floor near the player, they could also fall into lava, or get destroyed, or be lost in a different way.

Note 2: If the player is standing on a half slab, or inside of any block, then that block will be set to air, so it may potentially break parts of your map