how much should i be feeding 3 small diano zebras?

Twice a day is fine if it's convenient for you, just feed less per feeding than you would if you were feeding once a day. A good rule of thumb to remember is that a fish's stomach is about the size of it's eye, so it takes a lot less to fill it than you think. Make sure all the food you put in is being eaten within 3 minutes or so. If not, give a little less next time.

Once or twice a day is fine. Try not to get extra food in the tank because it may rot and cause ammoina problems.