Given that the Baltics are part of NATO, and NATO maintains a multinational advance force in the Baltics (to give the non-Baltic NATO countries some skin in the game), invading the Baltics wouldn't risk starting WWIII, it would be WWIII.

(Depending on the surrounding circumstances, it might not be the "start" of WWIII. If Russia invaded Estonia tomorrow, WWIII would probably be seen as starting with the Russo-Ukrainian invasion of 2022.)