My baby is 3 yrs old and a dr prescribed cetirizine 1mg/ml syrup (2.5ml once after dinner ) and my husband gave him 1 1/2 tsp instead by mistake...?

Will be OK...: Taking 3x the usual dose should be ok. Your child could get a little sleepy (or perhaps even a little irritable) but it is not harmful. If your child is taking many other meds or has serious underlying health concerns, i would probably call your poison control center (open 24/7). They are always helpful and reassuring to speak with.

Cetirizine: Totally safe. Nothing to worry about.