Guinea pigs will not eat their young, hamsters might, but guinea pigs are not hamsters,
. They will eat solid food, the same as what you feed the mom very soon after they are born. Are you feeding Alfalfa based pellets?
The normal gestation period is 50 something days to 72 days.
How old are the babies? Because if the babies were just born their mom could not of been pregnant when you got her.....Have you gotten the two adult pig's sexes confirmed by a vet?
By the way, if any of the babies are male they could get the two adults pregnant at 3 weeks old.
Congratulations of your 3 healthy babies! Do you have any pictures yet?