I "lock" all my button lights by unscrewing the head by about half a turn. Haven't tried it with a CX3, so make sure the o-ring is far from being exposed.
I just tested that and I believe it is not an option.
The tail cap is spring loaded and the light stays on even after unscrewing the tail cap pretty darn far.
Bummer. I was really hoping that would be a workable solution.