I m not able to breathe through my left nostril from past 3 years .... any suggestions?

Polyp: There are many possibilities. One of them being a nasal polyp. Some people also have a deviated septum. I would seek advice from an ENT if you have not already done so.

Clogged nose: Something is obstructing your passage. It could be a polyp, deviated septum, or a host of other things, none of which are serious, just discomforting. The only way to know and get it fixed is to have a doctor take a look with a scope, probably an ent.