See as the choices you have given , it's like chosing between Bugatti, Bentley, BBT. All three are great.

It's you who have to decide which branch is best for you.  In between IIT Guwahati and IIT BHU ,i would prefer IIT Guwahati electronics as IIT Guwahati is among the old IIT's , so neglecting the infrastructure, what i see is that the placements and other factors are better than that of IIT BHU.

Now Between IIT Guwahati and IIT Roorkee , IIT Roorkee has a slightly upper hand But it is almost same. So you have to decide . If you prefer Mechanical then IITR Mech.>IITG Electronics>IIT BHU Electronics. Else just reverse the first two. This is what i think. 

TThank you.