Oh boy. This is very off the top of my head. I'd need to fire up my HS tomorrow and check. BUT IIRC, when you've "gotten away" to start MB, the ball that made the ramp shot locks in the LEFT lock. Game should then puts a ball in shooter lane. You plunge up the ramp and it diverts to the right lock. Then puts a third ball into the shooter. Once you plunge that ball up the ramp, IIRC it launches the left lock, it diverts the ball to the right lock and launches the right lock ball BEFORE the third ball falls into place. Then launches the third ball out of the right lock.

I've had mine bug out where I plunged the second ball and it just started MB right away, launching the left lock. OR I've had the diverter miss diverting the second ball, sending it all the way around the ramp and drop back onto the playfield, where multiball starts once another switch is hit (without a third ball).

You can also start MB mode by re-lighting all the stop-light targets and then shooting the EB hole to "get away". Then your second ball gets diverted to the left, third ball to the right. Then it launches the two locked balls (I wanna say left, then right) before launching the EB ball.