3150' is your height above the ground with QNH set. You know that at 13D you should be 3900' feet above msl (13 x 3(00)). The difference should equal the elevation.
Feel compelled to post, although I'm sure everyone's heartily fed up with this by now.
remoak, after nine days of pondering the answer to a simple altitude/dme glide-slope cross check question, you're still posting complete garbage on the subject. Perhaps you're not actually a pilot, perhaps you're deliberately having a laugh ... or perhaps you truly don't 'get it'. Am I alone in finding that thought really scary?
What you've said here is completely wrong in every respect. 3150' is
not your height
above the ground with QNH set, for that you'd need to know the elevation of the local terrain. Instead, it's your height
above the airfield if the altimeter is reading 3,900' with the QNH set. BUT since, as an approximation, you should actually be at 4,650' at 13d, what on earth is the use of knowing what your height above the airfield would be if you were at some different altitude (and if there is a reason, why do you want to know what it is at an altitude of 3,900' rather than, say, 4,300' or 2,700' or any other random altitude you could select)?
Then, as has already been observed, the 13d cross check is 3,900'
above the airfield,
not above msl.
Nine days and still getting it completely wrong, allegedly because of lack of sleep! I truly hope that all the tired pilots I fly with can get it right in not much more than nine seconds.