I feel like it's important to point out that the modern approach to diet of picking one nutrient as a magic one, is, well, kinda magical thinking. Omega-3 fatty acids, beyond the minimum amount your body needs, haven't been shown to be a cure for anything and supplementation hasn't been shown to improve outcomes or performance. You need it, yes; but you probably don't need to build your diet around it. e.g. from WikiPedia:
"Dietary supplementation with omega-3 fatty acids does not appear to affect the risk of death, cancer or heart disease. Furthermore, fish oil supplement studies have failed to support claims of preventing heart attacks or strokes."
OK, so even if we assume we need more Omega-3 than a normal vegan diet contains, most meat also does not contain a good balance of Omega-3 to Omega-6. Eggs are only a good source if they have been supplemented with flax or other good sources of Omega-3. Fish is often a mixed bag, when it comes to health; it may be better to get our nutrients from the same place they get it. Seaweed and other edible ocean plants are a pretty good source of Omega-3 and are healthy on other fronts, too.
Finally, there are many normal foods that have a good Omega-3 to Omega-6 balance. Walnuts and other nuts, most seeds, several kinds of fruit and vegetables (while it's a smallish amount, the balance is good and you really don't need mega-doses).
In short, if you're eating a diverse vegan or vegetarian diet, you almost certainly don't need to think about any specific nutrient (except B12 for vegans, which does need to be supplemented, or obtained via nutritional yeast that has B12). You're just as likely to have specific nutritional deficiencies on a meat-based diet; folic acid deficiency is a common problem of a meat-heavy diet that would not trouble a vegan or vegetarian, for example.
A balanced and diverse diet is the right answer to questions about food and nutrition. Laser-focus on any one particular nutrient is probably counter-productive, unless there's a specific reason to focus on it (some people have difficulty processing some nutrients, for example, but Omega-3 isn't one of them, to my knowledge).
In short: Omega-3 is the new fad nutrient. There will be another in a year or two, and the entire health nutrition industry will figure out ways to sell you books, and pills, and diet plans, and the food industry will figure out how to cram more of it into foods, and we'll find that, like most of the other supplements over the years, it doesn't really do anything. You need some to be healthy; you don't need gallons of it to be healthy.