My MacBook Pro makes three beeps during updating
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.