If you get the YN 603 then make sure to get the Mark II version - they improved a couple of issues.
As mentioned above, if you're going to use eTTL then you need the 622, if you decide to stick with manual only then either the YN 603 II or the YN 560-TX. The 560-TX is a bigger unit, but allows remote control of power and zoom with the Yongnuo 560 III flashes (in case you plan to expand later). It can also trigger any YN 602 or 603 flashes (but you'd have to get an extra 603 II transceiver to work with your 568).