So I found that if I crossfire 3 RX580 8GB, I get 24GB of HBM2 Video Ram vs 22GB of GDDR5X or GDDR6 Video Ram. Which is better, SLI GTX 1080 Ti/ NVLink RTX 2080, or 3 way Cross Fire RX580 8GB?
I pay 600 for the RX580 3WCF vs about 1200 to 1600 for the GTX/RTX SLI/NV.
Sli is the worst one as you need a specific mobo to use it, crossfire works with mobo with enough pcie lines but NV is the best since it is able to use all the vram that both cards use instead of just one card being the main source for vram while the other card doesnt uses it own vram.
As to answer your question NVLink RTX 2080 due to being able to combine both vrams into one.
not true. crossfire and SLI do not pool VRAM like that. if you SLI or Crossfire say, 3 8GB video cards, you will have 8GB of VRAM available. if you SLI two GTX 1080 Ti's, you will have 11GB of VRAM. in general, SLI and Crossfire have poor scaling. I do not recommend doing this unless you play only games that have good scaling and you find that this is a good choice for your budget, instead of one fast card. an excerpt from Ubisoft:
"In Multi-GPU setups the VRAM is replicated, it never stacks up. For example:
Installing 2 video cards with 8GB VRAM each in an SLI/Crossfire setup will not result in an available VRAM of 16GB. The available VRAM of that setup will be 8GB."
SLI and crossfire as other's mentioned does not scale in terms of video memory. Reason being, the exact same information is loaded into each GPU's VRAM hence VRAM usage will also be identical. There are some modes which are not SLI in which VRAM does scale, but that is not important to the gaming arena and SLI / Crossfire.
However this is largely secondary as being honest the biggest question mark is over your relatively random choice of GPU's. What exactly are your trying to achieve here? Generally speaking, VRAM outside of a few very starved / low end cards is a relatively small point to consider for gaming as most mid range up, including ones you mentioned have largely enough Vram for the job and will usually run out of GPU grunt before VRAM.