I reckon to add variety to the game play in a small way is have the three pop bumper mounted to a mini playfield that can rotate say 30 deg from the start position on the 2nd ball start up and then a further 30 degrees for the 3rd ball. In rotating the mini playfield one way, when ball exits out of the 3 pops on one side could lead to a lane not accessible on the first ball or third ball and then when playing the 3rd ball it could direct the ball to like a ball pop up ejector to a awesome game toy - like a jet that shoots the ball to a inaccessible ramp on ball 1 and 2. This way there is something slightly different with each ball. These slight changes are goals to achieve through the game.

In addition maybe a lighting feature where the lighting represents a day and night effect:
1st ball - is like sunrise with lots of yellows and orange light
2nd ball - is like middle of the day - everything lit up
3rd ball - is night time leading into a darker playfield (less lighting) and maybe a black light.
if the game was set to 4 or 5 ball the lighting mode just repeats to sunrise etc.

I reckon having some variety in a game could help a story line and also add some change to game play and not get so repetitive.

Lastly if you like a particular feature you could over ride the game play with selecting the feature in the game settings then spice it up later with a different mode - inaccessible ramp with sunrise lighting mode.