Level per monster kill?

[Hexsyn]

I played my 1st game of Munchkin loved it!!! I had a quick question. I kicked the door Office Zombie turned up I could defeat. A player added wandering monster with nurse which i asked for help. Then another player played with a big whistle along with pizza guy luckily no other monsters were added and we were able to raise our combat strength to take care of it.
My question is would I get 1 level or since there were 3 monsters 3 levels?
If the answer is 3 what about my helper (Because of pizza guy)?