nice answers!

I'll just that extra copies of a single autosome (so 3) generally end in embryonic lethality, except for Down Syndrome and a few others very rarely. So dosage is generally important for whole chromosomes.

Extra X chromosomes lead to relatively mild phenotypes vs extra single autosomes, which may relate to X inactivation.

Total genome duplication isn't limited to just plants, and some frogs have up to 12 sets. I don't know a ton about that, other than that often many of those chromosomes aren't fully functional.