Computer tries to start but shuts down after 2-3 seconds

That's the cause of many a common fault and you may get the same with
the RAM cards n' CPU for that matter.. these are the only items
required for the initial bios boot, any of these will stop the bios
boot process.

Supposing all was well with no hard drive connected... it would say,
"Please insert your disc with the operating system on" or
simply "No hard drive found" or something to this affect.

The edge connector contacts are very fine and usually gold plated, all
you need is a little oxidization on the contacts and it becomes
'resistive' or not make connection properly... this usually happens
with age the contacts being soft copper usually becomes pitted after
being in some considerable time, this will cause intermittent
connections.

I recall the RAM slots on my Gigabyte mobo... they were unusually
tight and required a fair amount of 'hard push' for them to clip in
place yet the other cards went in with ease.

You learn hell of a lot from building your own, far better than buying
a OEM machine in many way's. Never even thought of suggesting to
re-seat the cards.

Good luck and good computing.

Davy