Is it ok to take 300mg x 3 cod liver oil tablets a day as a source of dha, epa and omega3? Any side effects? Benefits significant?

Unknown. Cod liver oil and other fish oil supplements are fairly good sources of o3fa of which dha and epa are two types. The exact dietary requirements are unknown. Se's are uncommon but some preps may contain contaminants. There are a lot of purported benefits to o3fa and dha in particular but they are still unsubstantiated . My advice would be to eat more fish on a regular basis and skip the supplements.
Fish oil . Fish oil tablets can lower lupus levels but can increase slightly bleeding risk as side effect.