Can one go to any of the competitive fellowships after a 3 year internal medicine/primary care residency?Or the chances are too slim?Similarly is it possible to change ones mind after some time of doing the IM Primary care residency.... say after 3/4 years,to a subspeciality fellowship?

yes and yes

If yes & yes.....then what are the fellowships that are easily available after a primary care IM residency?
thanks.

the same fellowships available to all 'board eligible' internists.
the competitive specialties will still be the same.
in no particular order and not inclusive- sports med, allergy, cards, gi will be difficult.