Well, as the camera isn't created yet, it doesn't have any coordinates, and that means you can't increment those coordinates with anything. 'Cause they don't exist.
Whenever you are creating a camera, create it at [0,0,0] then move it into the right place.
And to my knowledge, _camx/y/z will never work, as they aren't proper commands. Have to test that though.
For now, I suggest doing something like this:
_cam = "camera" camCreate [0,0,0]
_cam cameraEffect ["internal","back"]
_cam camSetTarget X
_cam camSetPos [#,#,#]
X = the unit/object you want to target
#,#,# = X,Y,Z coords. You can get exact coordinates by using camera.sqs (explained in the Editors Depot very well, look for Messiah's camera.sqs tutorial)