Move from the middle with a sidestep.

After half jumping move left or right, make sure there is no gap and jump directly.

After a few round coaches throw the ball to the hands of the blockers and try to find a gap.

Emphasize:
Look at the outside blockers position before moving, and make sure there is no gap.
The emphasis in this drill is not on blocking the ball but on the footwork of the middle blocker.

Variation:
Outside blockers should change position so the moving blocker has to adjust his side step(s).