Can I pursue B.Arch after 3rd year B.Com? How many years will it take to complete architecture after B.Com?

You can. There is no such restriction on studying architecture. Some architects concentrate on design work. Design involves both artistic and scientific skills, i.e. producing something which is also technically feasible.

Physics, chemistry and mathematics are no longer mandatory for admission to arc

Yes, you can pursue B.Arch after 3rd year B.Com. However, it is important to note that not all colleges offer lateral entry into B.Arch. You will need to check with the individual colleges about their specific requirements.

The minimum eligibility criteria for lateral entry into B.Arch is a pass in 1