Since baby us small u can give prebiotics rich formula milk like Aptamil.

This age of baby mother milk is good option of prebiotics.

Breastfeeding is the only option to boost immunity in a 3 months old

Mother's milk is like liquid gold for the baby. Till 6 month you should only feed breastmilk. If the mother is not able to breastfeed then she should give formula milk which is very close to mother's milk. Aptamil is very close to breastmilk and has prebiotics