Conversion rate of ALA to DHA is very poor, even worse to EPA. The longer the chain, the slower it gets..
Hunters are smart because they get more dietary omega 3 than herbivores. Double smart, because they obtain them without having to go through
laborious bioaccumulation. Fish oil or krill oil are best sources. Mercola cited today, a new study - admittedly on a small group - that demonstrated higher efficiency of krill oil in raising serum DHA/EPA. The additional cost is not justified, however - even if you want the astaxanthin (for which there ARE suitable plant sources)
All PUFA are susceptible to rancidity, and flax oil is notoriously unstable - same as linseed oil - a cloth soaked in linseed/flax oil will spontaneously ignite - sufficient proof of oxidation tendency if any proof is needed. Keep your fish oil refrigerated! Take care about your sources, too. Get it fresh, and check that it's free of pollutants.