Track IR 6 will not be out for some time after ArmA 3 releases, it's not dedicated to the ArmA series aswell ofcourse, it's worth you investing in the TrackIR 5. It will be a long time before your hear anything of the 6 being released.
Our very own community member dslyexci has a deal going on with natural point which will net you $20 off the final price. Check it out here
. More about the system with it's uses in ArmA 2 here
So with that discount you can get the TrackIR 5 and a TrackClip PRO for $164.90.
And yes it will be used in ArmA 3. The system isn't built around a game. You can use a Track IR 4 in ArmA 2, and you'll definately be able to in ArmA 3.