The center drivers were standard on all the PRO RMK's, RMK"s and Assaults for 2011/12 as well as the Dragons from 2008 on... Maybe you got lucky and they left yours off?
The 2008 155's got the two drivers... the 163's got the third driver.... 2009 All of them had it, std & Dragon.
Interesting thing I found browsing the online Polaris parts finder; only a few of the 800cc sleds (such as the RMK Dragon, Assault RMK) showed the driveshaft with the third anti-ratchet driver, but not all of the 800cc sleds had the center driver. The std RMK, Dragon IQ, and so on came with the 2 driver shaft.