This can be tough especially if you don't have a local partner because you'll be playing 1 vs 2.
What you want to do is keep all 3 balls by their net at all times. When you score and a new ball spawns, go over and hit it towards their net and keep them distracted. I made an achievement review/tips video for this game and I go over the arcade mode (at 2:05) and give a few tips that should help if your struggling.