How to beat the Three Mage Sisters Kirby Star Allies 2 player?

There are some basic strategies & ally setups that you can use here, along with the knowledge of how the 3 Mage Sisters attacks, that can get you past this difficult part.

First, there are a couple of Dream Rods & tons of powers in the area prior to entering the boss door. Think about your team setup before entering.

1. You should probably have at least one ally that can heal you. Whether it is Chef Kawasaki, Vividria, or Adeleine & Ribbon, anyone that can make food on the fly with help with survivability. When they are AI controlled, you can 'request' them to make food, if they are able to (no cooldown bar) by holding Up near them, like you were receiving an elemental power.
2. You'll want some 'Heavy Hitters'. These spots may be the best place to spend the Dream Rods. Whether your P2 takes one or given to AI, either way, they should be able to support greatly on damage. Some good choices that have high damage output and either large attack areas, long range, or attacks that let you move/defend after initiating them, are Daroach (long range beam attacks, bombs), Magalor (large sword attack, black hole, gem apple bombs, fireball launching), Dark Meta Knight (projectiles, several attacks that reflect), or Taranza (Taranza ball, Queen's Phantom!)
3. You may want to have some sort of defensive power. This may be a good use for Kirby. There is even a strategy around simply remaining a statue with Stone power through the fight, while allowing the AI to damage as much as possible, and only come out to revive allies. You can read more on that on the guide here. The Hammer ability also has some surprisingly defensive moves. The run attack (spinning hammer) has some great frames of invincibility, almost similar to guard dodging, that allows you to damage while also protecting yourself.

With your team set up (and tweaked as needed), it's helpful to know how the Mage Sisters will attack. They all have some slashing & lunge attacks, so it it generally safer to attack from behind if possible (they often fly to one side of the screen before starting their lunge or set of attacks). Zan and Francisca also have projectiles to watch out for (lightning balls, thrown axe), but are usually pretty avoidable. They also have a 'jab' type attack that attacks straight down (more often in the second half of fight), so be cautious of standing directly below. The difficult part is when they use their other powerful 'beam' type attacks, which they each have one, and often do concurrently.

• Flamberge has a fire cannon that shoots a large horizontal attack across the screen. It typically is done on the bottom half of the screen.
• Zan has a lightning bolt that also attacks horizontally across the screen, typically across the top half (on occasion, this attacks diagonally downwards across screen). This attack often occurs right before or after Flamberge's fire cannon attack, causing you to fly up or drop down quick to avoid both.
• Francisca has her water gun attack that creates a horizontal zig-zag across the screen that grows/constricts on zig-zag size. move horizontally to avoid damage as it grows/constricts, while also possibly staying to the vertical half of the screen you are stuck in if one of the other two is still attacking with their beams!

These beam attacks may come in a different order, especially after their first use, but often, the order of use is Flamberge/Zan/Francisca. You can check out this video to observe how each of them makes a certain part of the screen unuseable, and how each attack starts up so you can prepare for it coming. Each has some startup time before the attack starts. Flamberge lights the fuse on her cannon, Zan summons a yellow ring around her, and Francisca takes her water gun out and shakes it.

There truly is a lot going on at once in this fight, and you'll inevitably get hit. Just try to ensure you stay topped off on health (and share the health with your friends!), and try to avoid as much damage as possible (e.g. using guard dodges to help move across screen instead of just running). With some practice on what/when to expect moves to come up, you should be able to push through.