Isn't nh3 strong ligand (except for 3d +2 states), so it should cause pairing in b hence 1 unpaired e?
Nope. If there are only 3 electrons then one each will go in the t2g orbitals and there will be no pairing even if it's a strong ligand.
Is this an exception case??can you tell where nh3 is strong or weak?
No, this is not exception. I'm not saying NH3 is a weak ligand.
You're aware of t2g and eg right? T2g orbitals have less energy than eg. Even when a ligand is strong, if the metal is d2 or d3 configuration, no pairing will occur.
Okk ya I got it..thanks