I'm guessing they're wild? If they were in a nest, please put them back! Mama rabbits only visit their babies a few times a day so just because you didn't see the mother doesn't mean they aren't cared for. Chances are good that she will still accept them if you just put them back.
Please keep in mind that hand-fed baby rabbits- both wild and domestic- have a very low survival rate. There is no good substitute for rabbit milk (although some wildlife rehabers have some good specially-made stuff) and the risk of aspiration (breathing in the milk) is high.
If they really can't be returned to the nest because it's destroyed, etc., please contact a wildlife rehabilitator. Your vet or local Humane Society might know of one if you can't find any in the Yellow Pages. A professional wildlife rehabilitator can give these babies their best chance of surviving.