If your camera is not rotating / zooming in or out, you can just use mouse.x / mouse.y and set the layout scroll to the camera x/y position. You could also use absolute x and absolute y, but you'd have to shift the value. this is still basic vector math.
make two global variable which you set after moving the camera
// camera moving events go here //
q3dmousex = q3d.camx+mouse.absoluteX-windowwidth/2
q3dmousey = q3d.camy+mouse.absoluteY-windowheight/2
//////////////////////////////////////////// any events that need the Q3D mouse position come after.
if you only change the camera x and camera y position this will work, rotating the camera will require the vector math i was talking about, moving the camera will too.
The Q3D.sprite does what it does... its quite self explanatory. Have you tried using it? It's just a billboard for 2D graphics in 3D. it doesn't make 2D graphics 3D, it displays 2D graphics in a way which allows them to be layered with 3D objects in the scene.