Thanks Harry, but my logic board was replaced by Apple for the GPU issue already. And I don't get the beeps on booting High Sierra from the internal drive, just when trying to boot the Snow Leopard install disk. And the memory passed memtest run three times and the Apple Hardware Test, so it's not memory. I have no video glitches and the machine runs just fine, except for not booting the SL installation disk. As Dennis and Charlie have said, it is most likely a firmware upgrade that is now blocking SL from booting. The three beeps are just wrongly indicating a problem in the boot.
I did try running with one memory module, in both slots, but got no boot from that. No beeps, but no boot, either. I think I've eliminated the memory as the issue, which only leaves the firmware.