After 3 months of medication my endometrium lining still thicken it is 15mm. what is the right thing to do?

Depends: It depends on what is causing it to be thickened, what medicine you have used, where you are in your menstrual cycle and what if any problems it is causing. The doctor who has been treating you should be able to tell you what your options are. It also makes a difference if you have finished your family or if you want more children and if so when.

Endometrial lining: Usual dimension for endometrial lining is less than 10 mm. . Depending on the "medication", the lining can react to become "15 mm. thick" and still pose no immediate cause for alarm. There are conditions which would require a tissue sample, however, and you should consult your healthcare professional for advice.