Three-kills.
Mario is saying, I'm quite sure, "What a tasty treat." I wonder if, when the enemies are dead, he eats them... O_o
Luigi: "That's delicious!" What do Shy Guys taste like??
Hrrr... Toad's a bit tough to figure out. I think he's saying, "All righty! No mushrooms!" I'm positive about the "No mushrooms!" part, but I'm not sure about the first part. Good to know Nintendo is anti-cannibalism.
I heard Peach say "Fresh from the waffle," personally.
Oh, and when Toad dies, it sounds like he's saying "AAAAAAAAHHHHHHHHHHH!!!!!!!!!!!!!!!!!!!!!!!" What do you think?