I've developed a 3cm callus-like growth on the interior arch of my left foot. it feels like a callus, and isn't painful. what is it?

Lesion: The only way to know for sure would be to take a biopsy of the lesion and sent it for analysis. Aside from calluses, plantar warts are commonly found on the foot and may have a similar appearance to a callus. A good test would be to squeeze the lesion from side to side. Calluses are painful on direct palpation where warts are painful when squeezed from the sides. Seeing a podiatrist could help.

Depends on the cause: Many things can cause calluses. Some reasons that one may develop a callus include: viral infections (warts), friction, pressure, repedetive trauma, and a limitation of joint motion. Some genetic conditions may also cause people to develop calluses on the hands and the feet. Treatment depends on the cause of the problem. Have it evaluated by a professional and get the appropriate treatment.