I answered that yesterday already.
We used to build machines like that. Still have eight of them, in fact.
They're obsolete, replaced by cloud compute platforms that are so agressive on pricing to win your business that it's orders of magnitude faster and cheaper than local compute. If you can't do what you need on a single GPU then it's time to buy a compute farm or rent the compute. Maybe that won't work for everyone but the triple 3090 workstation is a pipe-dream of a niche market within a niche market.
Even if you can justify or desire a triple 3090 machine for some reason, you're going to want to split that load over multiple PSUs. There's a few of the streamers struggling to run 3090 SLI on a single PSU and that's not entirely the PSU's fault - it's an issue of domestic wiring in some places not supporting the kind of inrush spikes and power draw of a 1500W unit running off a single wall socket on a circuit.
Less of a problem in the EU with 230V and 13A breakers that can handle over 2KW without causing problems.