I would like to know if I could take Omega 3 tablets that have fish oil in them. there is a "kosher" brand on the market but I think that is because it does not contain gelatin. My question is , is the fish oil kosher ? I am taking it as a health supplement and not under a prescrpition from a doctor.

You should ask a Kashrut authority. Generally, unless it is medicine required by medical advise you will not consume anything that does not bear a Hechsher, especially if contains ingredients form animals. I can’t see how fish oil can be kosher without supervision as there are many non kosher fish around.