I've had calf pain in my left leg for over three years now. No swelling or redness but it hurts to walk on. Could this be a DVT after all this time?

Probably not. Dvt classically result in swelling +- pain (from a little to alot). Post -dvt syndrome can hurt for years after the DVT has resolved. See a doc ( vascular md if you are vry anxious about a dvt) and look for other causes .
Unlikely. I will do the never say never but highly doubt this is from a DVT - would expect swelling if there was a dvt. It is not impossible though.