Try one RAM stick one at a time in each slot and see if you can identify the bad slot or stick or both. Is the Power on LED flashing?
'UPDATE3/31/17
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Three long tones, three short tones, three long tones: EFI ROM corruption is detected, and the computer is in EFI ROM recovery mode.
Newer computers automatically recover from a corrupted ROM; when this occurs, a progress bar appears on the screen during ROM recovery mode. Don't disturb your Mac while the ROM recovery takes place. When recovery is complete, your Mac restarts.
If you need to restore the EFI ROM on certain Mac computers produced before 2008, you'll need to use a Firmware Restoration CD to restore the EFI ROM. Search Apple Downloads for the right CD image for your computer.