Macbook Air - Blinking folder at startup on occasion
Three startup tones indicates that the RAM installed in your Mac is not passing an integrity check:


  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. 


From: https://support.apple.com/en-us/HT202768

Misbehaving RAM can contribute to a host of problems, including, (but not limited to,) the startup problems you've been experiencing.