Omega 3 can be in found in vegan foods in flax seed, chia seed, or seaweed. It's very easy to add some chia seeds to rice without affecting the flavour.

B12 is available in nutritional year and fortified plant mills. The "natural" way humans for b12 was by eating bacteria in soil, but since modern farming techniques mean that humans don't eat much soil anymore, is best to supplement. It's more efficient to get b12 directly rather than filter it through the digestive tract of another animal before getting it.

Vitamin D can be synthesised by being in the sun, but since this is a guide for Canadians, and there isn't much sunlight for about half the year, they recommend supplementing. Vitamin D deficiency is very common regardless of diet

I hope that answers your question of how to get these essential nutrients on a vegan diet. I hope you found these tips useful.