I've built three setups like this so far:
We've got 3x 4090 (MSI Suprim liquid cooled) working on 1 power supply.
Off the shelf components with very slight mods.
AMD Ryzen Threadripper 3960X 24-Core
TRX40 AORUS PRO WIFI
256GB RAM
1600w Super Flower PSU
Fasgear 12VHPWR to 3x 8-pin power cables
3x 4090 SUPRIM LIQUID
Corsair 7000D case (this, or a similar case is critical)
OctaneBench Score ~3800
If you're worried about power draw on one PSU (hasn't been a problem for me so far) you can use MSI Afterburner and lower the power limit (to 70%-80%) with VERY little impact on the octanebench score.
Necessary mods:
-Remove the covers and fans from the MSI cards. Just a few screws, very easy and reversible. They make the cards just slightly wider than 2 slots. With a 140 case fan blowing across all 3, temps are just fine if not identical to leaving the blowers on the cards.
-low profile or right angle Mobo io cables might be necessary to fit the GPU in that lowest PCIE slot. I had to go with these USB 3.0 header extension cables, or you can just skip all your case front io.
https://i.imgur.com/LUhtgAL.jpg-The side exhaust rack of the 7000D is just a little too short to fit the 2 radiators on it. I had to cut off the bottom (where it bends back toward the interior) as well as cut or bend the mating surface on the case frame. This allows the 2 radiators to just perfectly squeeze into that side exhaust area. The third radiator routes out the top. The front is for intake.
https://i.imgur.com/wuAzKQD.jpg