My MacBook Pro makes three beeps during updating I would like to find out what caused my problem. A few days ago, I decided to upgrade my MacBook Pro to High Sierra, and somewhere in the beginning of the process, the computer suddenly shut down and then made three short beeps. And the interesting thing is, when I turned the machine off forcely and turned it on again, the installation continued like nothing happened. Now the same thing happened when I was installing the update that requiered the reboot. I would just like to know what could happen, and how I could prevent it in the future.
Three successive tones, then a five-second pause (repeating): RAM doesn't pass a data integrity check. If you recently replaced or upgraded your computer's RAM, check to see if it was installed properly.

Three long tones, three short tones, three long tones: EFI ROM corruption is detected, and your Mac 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.


From Apple Support.