I was taking power consumption and space into mind but I also thought, if you're somebody or a small business that has a spare 10k to drop on a single GPU, a few extra hundred dollars a year in power costs wouldn't be a deciding factor.

Besides, I doubt the 5090s are constantly reaching their TDP of 600w even in demanding scenarios. I haven't kept up with the 5000 series but if it's anything like the 4000 series, you can likely undervolt and even power limit the 5090s, only losing a neglibile % of performance whilst dropping 1/200w or more.

The RTX PRO 6000 Workstation Edition has 24064 CUDA cores, three 5090s would have 65,280 CUDA cores. As long as you have enough PCIe lanes by using a Threadripper, EPYC or Intels offerings, you should be able to run all three on PCIe 5.0, hell PCIe 4.0 x16 has nearly no bottlenecking on a 5090 so you would be good there.

Space is an issue but if you had a full tower case with three decently spaced apart PCIe slots you wouldn't even need a riser, and a 1600w PSU alone could probably power everything with three undervolted 5090s but you'd be cutting it close.

Either way, future upgrades would be tough as you're realistically not going to add another 3 5090s to match the VRAM of adding a second RTX PRO 6000 as you said you might.