Is 3cm thymoma tumor considered big? also, pet scan glowed warm, not red hot. is it better if it did not glow red hot?

No, it is early: This is considered small as most such tumors would be silent due to their small size. As for its glow, your sister has already given us the suv which is a better piece of information than the warm glow! this suv is not very high...This is often a good sign in terms of the degree of malignancy. So it may be a slow growing tumor. Yet it is going to require a biopsy to determine what it is.