Have a rash next to belly button a little bigger than a quarter for three years unchanged. it's not a metal allergy. could it be something serious?

Tough to Dx: Tough to DX rashes over the computer. Belly buttons are common areas for wet/moist rashes to occur including fungal infections to psoriasis like rashes. You should see a dr, best a dermatologist (skin specialist) to properly DX the rash and get the best treatment. Best wishes.

Psoriasis: I do not know of any rash that would stay for three years unless it is psoriasis.