Even farmed fish has enough omega 3 to be well and truly worthwhile eating. The non-fish omega 3 fatty acids, though still classified as omega 3, are a different molecule from that in fish, so not as beneficial, and flaxseed oil is said by many to go off too quickly to be useful. Flaxseeds and milled flaxseed are OK, though milled it should be consumed quickly once opened and kept in the fridge.
It does seem odd that nuts/cheese/yoghurt, once frowned on for their fat content, are now felt to be beneficial. The type of sugar in milk - lactose - is largely missing from cheese/yoghurt. Lactose is broken down into glucose plus galactose; the latter is used in animal studies to accelerate ageing! For this and many other reasons I try to avoid cow's milk, and make my own almond milk for my daily porridge.