I have a couple of spare sets of the rubber band Trumpeter KV-1 tracks and was thinking of using them to replace the rubber band tracks that come with Trumpeter's IS-3; Do I understand correctly that the KV-1, IS-2, and IS-3 all used the same track?

The IS family tracks, although very similar to those used on KV tanks, are narrower. The kit offered by Trumpeter is an example of post-war modification,thus it should have later type tracks.
Here is a link to all types of tracks used on IS/ISU family vehicles:

http://www.dishmodels.ru/gshow.htm?p=13119

HTH, Paul

From all the pictures I've seen all JS chassis vehicles used the same track, but the original KV-1 has slightly differant looking tracks compared to the former.