The title suggests you are using an X3 though.
It's odd just leaving oput a strip though. The spacing should be different if there are 2 or 3 strips and just leaving out the middle one wouldn't give you the correct spacing for 2 strips with too much light on the walls. Or vice versa, simply adding a strip in the middle of 2 correctly spaced strips would also give you uneven lighting in the middle.
The strips should be 12 inch spaced (middle to middle) for 2 strips and 8"apart with 3 strips.