Cyst&tooth#3 removed,maybe old root canal caused,ate thru nasal cavity bone.ct says maxillary retention cyst now,2 weeks ago 3dxray showed nothing?

Cyst: Maxillary Retention Cyst shrinks on its own in time. In your case, since the possible source which was an irritation/infection from a failing tooth when removed, the cyst disappeared as well 2 weeks post surgery as presented by a 3D technology. Isn't that good news?

Not unusual: Retention cysts should be followed up by your dentist or ENT clinically and by radiography. Mostly no treatment is indicated, because of their high spontaneous regression rate. Take care.