It reminds me of people who added Invidious links back in the day, but over time the individious instances would go down, preventing people from knowing which content was initially linked.
Yes, the archive.today links have that risk, but not the archive.org ones. Also, you can manually modify archive.today links to show the original link.
With the way this seems to works, can’t we just create thousands of links to let’s say Lemmy.world from all the other instances, to boost the LW domain authority? Shouldn’t all the crossposts links be taken into account in this calculation?
That might be one reason lemmy.world has a relatively high domain authority.
And if yes, then do 3 links here really make a difference?
Your 3 links won't make a difference. Starting a trend and spreading the word will.
I'm no SEO expert, but search engines penalize websites for gaming the system. I've already read some suggestions that Google is not sure what to do with the fediverse because it already looks like spam, and that may be why it doesn't show up often in search results. That's beyond my knowledge.