Pedialyte is generally used for toddlers 1 year and above but sometimes pedialyte can be given to 3 month old babies but in small amounts.
You should consult with your babies doctor before giving them pedialyte and to get the correct amount that you can give to your 3 month old.
But generally you can give your 3 month old baby half a baby bottle of the pedialyte at a time but avoid giving them too much because it could cause further health problems.
Don't give the baby the pedialyte all at once but instead feed them a little bit of the pedialyte and then wait another 30 minutes or so to give them some more.