OK. It depends on which BIOS it has.
If it is an AMI BIOS or PHOENIX (very common), 3 short beeps means "Base 64K RAM failure". IE you have a RAM problem. Could just be that a memory stick has become dislodged or may have gone kaput. Try re-seating the memory sticks (if you are competent to open the PC). With a PHOENIX BIOS this would mean row 1 is the one, but AMI does not tell you which one is the problem.
If it is an AST BIOS or MYLEX BIOS (not very common but possible), three short beeps means "Keyboard controller reset failed (POST 3)". Probably the motherboard is faulty and unlikely to be fixable.