Mac Pro 3,1 does not post

I purchased this 2008 Mac Pro and I’ve been looking to diagnose what’s wrong with it for the the past couple of days

It powers on, all fans come on and then it does nothing

What I’ve done:

Removed gpu and replaced with a PC one (tried a GT 240 and an 8600gt), I’m aware I wouldn’t get a boot logo but it would give me the lock screen if it was working, but nothing

Removed the hard drive and hooked it up to a PC, crystaldiskinfo reported it as being in “good” condition

After that I removed the RAM and swapped out processor A to processor B’s place and vice versa to check if it was CPU failure

Ran them without heatsinks and the motherboard did detect they were overheating, so they’re probably getting power hence maybe it’s not the power supply’s fault?

With only the CPUs and nothing else connected to the Mac I attempted a power on and I got basically nothing again, no flashing power light

Diag leds “Standby” and “Power Good” turn on when I press the button for them, this both with nothing connected and with RAM, HDD and an 8600GT connected (only 1 stick in each riser card now)

I am not an expert in Macs and I have no idea what is wrong, I’d assume the motherboard at this point but I can’t be sure since I’m not very knowledgeable on this topic