But now I can calculate (and plot) the total angular momentum of this ball-spring system. Actually, I can't plot the angular momentum since that's a vector.

When you bounce a tennis ball off a moving basketball, ... Momentum isn't conserved in the three-ball-drop case since I didn't include the momentum of the Earth.

Summer Fun: How to transfer momentum from a basketball to a tennis ball Dave Maiullo, Physics Support Specialist at Rutgers University, does a very simple demonstration using just a tennis ball ...