is it necessary that d2sp3 is always diamagnetic. and sp3d2 is always paramagnetic. in octahedral complexes.if the answer is yes is [Cr(NH3)]3+ diamagnetic or paramagnetic as it has d2sp3 configuration.

No, magnetic nature depends on various factors like  nature of ligands as the weak ligands can not pair up some unpaired electrons and so the product becomes paramagnetic or sometimes in presence of strong ligand for same system, the electrons are paired up and product becomes diamagnetic.
The [Cr(NH3)6]3+ is diamagnetic as the Cr+3 ion has d3 system and all the electrons remain unpaired and the hybridization is done by adding rest two d-orbitals and it becomes d2sp3 and so the compound is paramagnetic.

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