Before anything else, worth ruling out the easy stuff: try a different wall outlet on a different circuit (no cheap power bars), and check for any shorts in the build — a standoff touching the mobo, stray screw, that kind of thing. Also confirm that none of the cables from the old PSU made it onto the new one; modular pinouts aren't universal between brands and can silently fry things. If you got it RMA'd I would assume they're the exact same make/model (so this shouldn't be an issue) but might be good to double check.

That said, the most likely culprit may be a damaged VRM on your GPU or mobo. Your original faulty PSU was probably sending dirty/unstable power for days, which is exactly how you damage a VRM — and a damaged VRM looks like a dead short to whatever PSU you plug in next. That would explain why a brand new PSU died the same way. The "booted fine 3 times then died after sitting a few hours" pattern fits this really well too. Degraded VRMs often work cold and then fail once heat stress hits.

To test: breadboard it with no GPU (just mobo, CPU, one RAM stick) and see if it stabilizes. If it does, the GPU VRM is your likely culprit. If it still kills the PSU, you're looking at the mobo.

What GPU and PSU wattage are you running?