Why did my MacBook screen turn black and start beeping three times?

Try to turn it on and hold the Cmd + R keys and see if you can start it in Safe Mode

3 beeps are indicative of a ram failure.

Have you changed your RAM sticks or anything recently?

what happens if you turn it back on? do you keep getting the 3 beeps or does the machine sometimes start up normally?

You can do a apple hardware test to see if it detects anything wrong with your RAM. Using Apple Hardware Test - Apple Support

If the hardware test can't detect anything the problem may be gone (or it may remain undetected by the apple hardware test)

However, back up all your important stuff to prevent any data loss.

If the problem persists:

Depending on the model that you have you can acces the RAM sticks and take them out. if your model is before 2013 this is the case. You can take out one of the RAM modules and see if the machine boots. also try this with the other RAM module. you can then verify if the problem is in one of the ram modules.

Hope this helps!

Based on your description, your MBP had a kernel panic, probably due to a RAM problem. Log and post it here per these instructions:

https://support.apple.com/en-us/HT201753

Ciao.