Complete the indicated task to get multi-balls on the indicated stage.

Stage 1: There are two sets of worm-like drop targets on the lower board. Get multiple hits to push them all the way in. After all three on the left are done a gray caterpillar will appear under the top flippers. Hit it once. It rolls up into a ball and becomes a multi-ball. It will unroll if you lost your original ball.
Stage 2: Hit the ball in the ring in the corner and get all the yellow pellets.
Stage 3: Kill the snake. The doors it uses to enter and exit the board will become lock shots, allowing up to four balls to be held. Shoot around the right loop on the main board (with the "5000"s) to release them all.