If I can't eat tuna for its mercury toxicity, what can I eat to have omega-3 since I have RA and taking methotrexte evry saturday & folic every sunday?

See details. Studies have conclusively shown that mercury levels in fish are too low to cause any issues in humans.
Wild salmon,sardines. Tuna is one of the fish with relatively high levels of mercury but many fish are high in omega 3's ; low in mercury including salmon and sardines; avoid farmed salmon due to pcb's. Or you can take fish oil capsules- make sure it is purified to remove mercury and ideally esterified to improve absorption. Flax and hempseed oil are also good. See http://www.Nrdc.Org/health/effects/mercury/guide.Asp.