Is pure 100% coconut oil good to use on a 3 month year old baby hair?

Unlikely harmful: While there really isn't an "scientific evidence/studies", it's unlikely that coconut oil will cause any issue unless it's the rare case that your baby's skin may be sensitive to it.

Skin and coconut: Coconut oil is moisturizing and boasts antifungal, antiviral and antibacterial properties. It is safe to use on the skin and scalp.