I have been treated for cushings. Can the tumer return? It has been 3 years. It was a adth pruducing tumer if the pituary.

Yes. Yes, it can recur even after many years. It is still difficult to predict which people will have a recurrence, but re-evaluation should be undertaken if suspicious symptoms develop.
Unlikely. Usually if there were able to remove the whole tumor is very unlikely that it will grow back, but needs continuos follow up.