Stick with the 2. I acquired an ambit 3 and the only difference between the two is the bluetooth feature. It allows you to sync your "movements" with the Moveslink app on a smartphone. The Moveslink app gives you a cool overview of your workout, but I'm pretty sure you can do the same thing on a PC. Also, the bluetooth allows you keep it continuously synced with your phone so you can receive text alerts and other stuff I found useless. You'll kill the battery daily if you keep it synced.
If you like your ambit 2, there's not a lot of reasons to upgrade.