Essential fatty acids consist of omega-3 and omega-6. They are called essential not only because our bodies need them, but also because our bodies cannot synthesize them ourselves. Omega-9 is biologically important, but the body can manufacture it, so it's not essential that your diet includes omega-9. BTW, olive oil is largely oleic acid, an omega-9 fatty acid.
Although omega-6 is an essential fatty acid, we commonly consume too much rather than too little omega-6. Too much omega-6 versus omega-3 is unhealthy. So, it's better to limit, rather than indulge in, omega-6.
Omega-3's have three main varieties: ALA, EPA, and DHA. Plant sources, e.g., flax, walnuts, are almost all ALA. The body needs to convert ALA to EPA and DHA, which is why some may disparage the effectiveness of plant sources in comparison to fish. People can generally convert ALA to EPA and DHA, but some people supposedly lack such ability. Fish omega-3 is EPA and DHA, which fish get from algae they eat. The downside of fish, of course, is possible mercury, PCB, and dioxin contamination. Fortunately, (at least as I understand it), cold water fish such as salmon contain fairly small amount of these contaminants. I'm a believer in eating wild salmon vice farmed salmon.
I'm pretty sure the flax-seed-causes-prostate-cancer is nonsense. Could someone cite the study? Flax seed (before it's grounded) has a protective shell that protects the omega-3 fatty acid, which is quite delicate. Therefore, you want to buy flax seed that's not yet been ground up. However, to make it digestible, you do need to grind it, which can be done with an inexpensive grinder. Just grind only what you are going to consume at that time.
Regardless of how you get your omega-3, remember that omega-3 is a delicate molecule. Time, light, and heat are all enemies of omega-3. Unfortunately, when omega-3 decomposes, it doesn't just cease to be the desirable fatty acid you need, but it becomes a harmful oxidation product. So, consume your omega-3-rich foods while they are still fresh, and be careful how you cook or bake omega-3-rich foods. With fish, you need to kill potential parasites (e.g., worms) while not killing off the omega-3 acids. So, don't cook/bake your fish longer than necessary. (On fairly rare occasions, eating fish raw, e.g., sushi, could result in parasitic worms.)