What and When do i feed my 3 mollies?

i have a 2.5 gallon tank and it has a white,black, and dalmation mollies in it. The person at the pet store said i sould feed them these little pebbles as food. But..... on the can it says i should feed them 2 to 3times a day. So i feed them once and all the pebbles sank to the bottom of the tank. So then i tried flakes and the mollies ate them right away. SO should i feed them like this 1 or 2 times a day or what?