What is the reference plane for the ILS glideslope angle? Is a 3-degree glideslope based from the runway surface slope at the point where the antenna is installed or is it referenced to a zero-degree plane?

If you know of a design document or FAR/AIM reference, please include that as well.
It is referenced to the zero-degree plane, and depending on obstacle clearance it may be a little steeper (3.5° or 4°). There is no reason to refer it to the runway slope as it defines a sink rate to speed ratio during final approach.