Not so sure about his third grip. Looks almost eastern backhand to me, or a weak eastern backhand, which is great for top/slice and kick serves, horrid for flat first heater serves.
Remember BorisBecker? He is a pro, uses a conti grip close to eastern FOREhand on every serve.
This grip thing. If someone says ONE way is the correct way, shut his audio down right away!
The correct way, for you and you alone, is the grip you hit hard, you hit consistent, and you can replicate it over and over again. Every player is different.
To me, grip 2, what I consider a conti that can be used for all volleys, is the ideal first serve grip. #3 is a second serve grip.
Milos would argue, but generally, and with Becker in mind, I think I'm close to the grips most pros use.