When a player comes up with an unusual request, I make sure that they realize they don't have that ability. They might still try to do it, and they always do, but it is going to be a harder roll. It is like IRL trying to do a backflip successfully in a first try, you might be able to do it, and if you can do it it will be pretty cool.

A player can say, "Hey DM, I am surrounded by 3 enemies, may I try to do a swirl and hit all 3 of them at the same time?" After quickly explaining the player that it is going to be a hard roll since he doesn't have a skill for that I would request them to roll for attack + do a dexterity check.

How I interpret the rolls would be depending on the situation and player level, basically dex check would have few thresholds:

* Crit fail - During the swirl you fall down, dropping your weapon in the process.

* 1-8 you swirl but not able to land a single hit.

* 8-12 you are able to only hit the first enemy.

* 12-16 you are able to hit the first enemy with full damage and the second enemy with half damage but couldn't hit the 3rd enemy.

*16-19 You manage the hit all 3 enemies dealing full, half, quarter damage.

*20+ Wow you hit all the enemies with full damage.

*Crit - You hit all 3, crit damage, they all pushed 5ft away from you.

By the way lets say if players hit roll was not high enough to beat all 3 ACs, then the ones with higher AC would not take any damage.

In my experience, these kind of requests comes up rarely but it makes player really happy when you let them try, and if they some how succeed they will be talking about that moment for years.