In the code you already have, src
is given a 3 element Array containing the X, Y and Z values of the Camera position.
What you want to do is take the X and Z from that and put your own Y in. So, construct a new 3 element Array, where the X and Z elements are picked from the existing Array like this:
src = new Array(thisComp.layer("Camera").transform.position.value, 50, thisComp.layer("Camera").transform.position.value);
Where 50 is your own value for y. Could be another variable also...
Hope this gets you on the right track,