I have a bump/sore/blister on my groin, just in between my thigh and genital area.. size of a dime. after three weeks, it has not changed.

Could be infection: Please have a doctor look at it. Syphilus begins as a painless ulcer. Other skin infections can cause a sore blister or papule bump, too, like folliculitis, infection around the hair follicles. MRSA is out in the community now, not just the hospitals, so please be safe and see your dr. Herpes can be an extremely painful blisters/ulcers.