If you don't brush your teeth...
"...your teeth will fall out."
"...the sugar monsters will come and eat your teeth when you're asleep."
"...your teeth will be filled with black holes when you wake up."
"...no cute girls will want to kiss you!"
"...you won't be able to chew and you will have to drink all your food like a shake!"
Ironically, the prospect of any of these happening is way too interesting to pass up for a 3-year old. Too bad bribing him with M and M's sends the wrong message.