Disconnect BIOS battery and every source of power for about 5 minuts. After this time insert again battery, connect to power and try to start. If this doesn't work that motherboard is trashed.

Here is some more info about this 3 beeps:
http://www.bioscentral.com/beepcodes/amibeep.htm#
3 short - Base 64K memory failure - A memory failure has occurred in the first 64K of RAM.  The RAM IC is probably bad

Try start it with other RAM.