...I figure it's just like the suspension on a car (it looks different ,but the results should be similar), the same tests apply.
trailer suspensions were more like a car. In practice, they are cheap, stiff, short-travel, and underdamped compared to any acceptable car or light
truck. Not that I have a strong opinion on this subject
The 3" (roughly) deflection is from no-load (jack up the trailer so the wheel is not supporting any weight) to full bump deflection. From no-load to full load (2000lb axle loaded to the whole 2000lb, even if the trailer is only 1000 lb by itself) is more typically only 2" for the axles in smaller trailers. For details, see spec sheets such as the Dexter Torflex 1000-2000lb Application Information
So how much should the trailer move vertically when you jump in it? Well, a 200 lb person stepping in (not jumping) should move it a small fraction of the amount which 2000 lb of full load does, so it will be a small fraction of an inch. Jumping might double that, I guess. The tires
probably give about as much as the suspension - but that suspension travel is still important.
I have a leaf-spring beam axle (not rubber torsion) but the situation is about the same. Moving about the trailer (without stabilizer jacks in place) causes noticeable movement, but I don't how much is suspension travel.