What's to be done about calluses that have been there for 3+ years?

Have them debrided. Callus are caused by pressure and will continue to return as long as the activity causing the pressure continues. Calluses on the feet can be shaved by a podiatrist.
There is a reason. obviously the reason as to why the calluses keep returning has not been addressed, at least successfully. Calluses form because of uncontrolled pressure on tissue. The tissue responds by hardening for protection. You must determine the cause of the pressure.