I swing with a 5/5 vanilla creature my opponent blocks with 3 2/2s I then regenerate my creature. Do 2 of the 2/2s die or is no damage dealt bc it's now removed from combat?
Regen doesn't prevent damage. Regenerate is when the creature dies: you remove all damage on it, remove it from combat and tap it
So to answer your question yes their creatures die too. Unless they all had first strike, then your creature is killed first and removed before it has a chance to do damage.
Ok cool thanks!!