rd2002

if you hear 3 short beeps, the problem should come from the ram, assuming your power supply provides enough power. so, try to troubleshoot the rams one by one. i know it's gonna be time consuming, but it is how you fix pc.
you also mentioned the issue related to your mouse and keyboard. i think you might want to check whether you connect the mouse and keyboard to ps2/usb ports properly. if you can confirm the connection isn't the issue, i think your motherboard should be the responsible of your problem.