I think I got it.. But I dont know how far you will appreciate my answer.. This is what I've written in my class notes.
Confusion is in 2nd and 3rd right? We have certainly decided that 1 is least basic. But for 2 and 3 we have to compare right. So what I understand is that methyl groups are sterically hindered.. So here let us consider the methyl groups as two halves with a plane passing vertically (just imagine for explanation). Now observing LHS methyls... one will move out of plane due to steric hindrance while other remains in same plane. In RHS, same thing will happen. However, now one methyl from LHS and one methyl from RHS will be in same plane thus resulting in further steric hindrance thus destabilising the molecule slightly while this destabilizing effect is not observed in option (ii)... Okay let alone destabilizing now these methyls are trapped therefore donation of electrons is present but comparatively poor.
My two cents. But I'm not sure if I'm correct. Actually I read about SIR only after I went through this thread coz we were taught most of concepts through questions.. Anyways correct me if I'm wrong please.
@Supreeta_Sen @Chirag_Hegde please see.