At least in Europe, protein extracts like pea protein are much more expensive that chicken meat, e.g. 5 times more expensive.
I do not believe that paying such prices is rational, so for many years I thought about eating only vegan food, but I could not see any way to get enough proteins without gaining weight.
I could switch to vegan food only after I have discovered that I can make a home-made bread that is highly enriched in protein by removing about 75% of the starch during a 6 to 7 minute washing of the dough. With a longer washing time it is possible to separate completely the gluten from the starch, but I am not willing to spend more time every morning with breakfast preparation, and this is enough to provide on adequate protein to calories ratio.
For Omega3, the only good alternative to fish oil is Schizochytrium oil. A few years ago, it was 8 times more expensive than fish oil, but the price has been declining slowly and now Schizochytrium oil is only 3 times more expensive than fish oil (e.g. with taxes and shipping included, EUR 300/L for double concentration Schizochytrium oil vs. EUR 50/L for cod liver oil). This is still very expensive, so hopefully the price will continue to decrease.
Vitamin D3 is normally made from sheep wool, which seems a perfectly sustainable source. There exist products which claim to contain "vegan" D3, but until proven otherwise I believe that this is a scam for extracting money from naive vegans.
The "vegan" D3 is claimed to be extracted from some kind of lichen, but it is irrational to pay money for such a product unless the vendor discloses which is the exact species of lichen, whether it is a wild lichen or a cultivated lichen, how it is cultivated and how the vitamin D3 is extracted from it.
If the lichen is not cultivated, but wild, that is not acceptable. Wild lichens grow extremely slowly and exploiting a species for vitamin extraction is guaranteed to cause the extinction of that species in a short time.