How do I

That's pretty much how you'd do it using the physics behavior. Set gravity to 0 and use linear damping to slow the balls down over time. One complaint I've seen is the collisions can be spongy at times. You can improve that by setting the iterations up higher in the physics settings.

You could use the bullet behavior instead but multiple balls bouncing off each other won't be handled well.

A third way would be to do to custom made physics. This is the very advanced approach, but can yield more precise physics. The ultimate would be to impliment 3D physics for the balls.

how would 3d physics work in construct plugins?

Well, there is no plugin for it, and I'd do it all with events. The main difference is it would allow the ball to rotate in all directions like a real pool ball would, so you could get motion near exactly like real pool balls. It would just be the motion, to make the ball look like it's rotating that way is another thing entirely. Look at the wikipedia page about impulse to see some of the math involved.