Some foods, such as flaxseed and other plant foods, contain the omega-3 fat ALA. Fish and other foods that are mostly animal-based contain long chain Omega-3 DHA and EPA.
ALA is a precursor to DHA and EPA, which means your body can convert some of it into these two omega-3 fatty acids. However, the conversion rate from ALA to long chain fatty acids is very very low.
Its best to focus on getting enough EPA and DHA directly from your diet or supplements, rather than by consuming ALA.
Fatty fish are the best food sources of EPA and DHA. These include salmon, herring, trout, mackerel, sea bass, and sardines. It is recommend that you try to eat at least two servings per week.
You can take DHA and EPA supplements made with fish oil or krill oil. However, vegan omega-3 supplements, which derive the nutrient from microalgae are also available. All of these sources increase your omega-3 index.