Hi QWSA,
As I'm sure you understand the angle will reflect where the aircraft will command a 3 degree profile as opposed to simply maintaining an altitude constraint and then thrust idle. It must by definition therefore be prior to the FAF which is nominally midway along the approach according to ICAO requirements. If you want to know from which point the constant rate descent will occur look at the waypoint immediately above the -3.0 listing in the box, thats where it starts.
For a NPA, whether it be GNSS or otherwise it's important to understand exactly where the aircraft will be following a constant descent profile as opposed to an idle descent to the next waypoint.