Does omega3 help with ADHD symptoms?

Perhaps. There are a few studies that seem to indicate it may help some of the behaviors. As with any supplement there are possible side effects - stomach aches, loose stools, fishy breath and at high doses can have bleeding problems. Read article on comparison of brands. Many products are not made up of what is needed for the brain function you are looking for.
Yes for many. Many children with ADHD have insufficient levels of Omega 3 fatty acids, esp. DHA, and research has shown that supplementing with it is often beneficial. Use a form that is purified by molecular distillation- I like Nordic Naturals. See and for info on the research and other useful supplements.
Omega3 Tip. Vitamin Shoppe enterically-coated are highly rated, excellent potency for the size (even better than nordic brand which I love) and there is a small size that can be used more easily with folks and children who might have difficulty swallowing pills. The enterically coating helps avoid the fish oil "burps". Nordic naturals best for this issue though with the lemon flavor. All the best.
It may. It is a recommended supplement for brain health. It does not improve focus or concentration. Rather it may help to improve circulation and blood flow in the brain.