The theme answers are based on the number of balls in the clues.
20A. Many traditionally have three balls = Pawnshop signs
35A. What four balls may let you do = Walk to first base (in baseball lingo, “a walk”)
52A. Gizmo often with five balls that demonstrates conservation of momentum and energy = Newton’s Cradle (I recall seeing one of these a long time ago but never knew its name.)