Can I use an AC rated momentary switch as a PC power button?
The &quot;AC 250V 3A&quot; rating is the maximum that the switch can handle.
If you would use such a momentary button:

and connect it to the PC's motherboard then the switch would need to handle about 5 V at a very small current. This is a lot less than 250 V and 3 A so the switch can handle this perfectly fine, nothing will blow up.
I don't think you intend to do this but I'll mention it anyway: You should NOT connect this button to the AC mains power for safety reasons!.
So: connect to the PC's motherboard will work perfectly fine.