You can download a vmware image to test ISPConfig 3 and check out the differences.
If you have a lot of websites on it I would stick with 2 and wait till there will be an update from 2 to 3. Just to save you a lot of time trying to get everything to work
If you have a few websites just setup the new system and copy site by site by making new sites/settings in 3. Only takes some work changing path and database differences.