Can anyone tell me how the best 3D flyers grip the sticks? Thumbs only or Thumb/Index finger combo?

Some of each. The best flyer I have seen in person is Sean McMurtry. He holds his radio tightly, pointed down and pinches the sticks. If you watch Quique Somenzini fly on a video he is a thumb flyer.

David

thumbs i think what ever way u learn or fly is what the answer will be

Users preference - if you learn one way it is very hard to change.

I pinch the right stick and thumb the left stick

Pinching the sticks allows a much more precises movement because you have two points of relation with your thumb and index finger, not just one like with your thumb.

Stick 40