Denise, first I would not give them science diet, it's really overpriced and a very bad food - do a search on science diet on the forum to understand why.
Second, when they don't eat everything in 15 minutes or so, you should just remove the bowl until the next feeding.
That being said, my pups have always had only 2 meals and it was never a problem (lots of treats in between for training though). More than two and they wouldn't eat everything either (well Boris, Tips would eat 6 meals a day
).