Xnr,
They are usually determined during the IFR test phase, and are the result of the steepest ILS gradients found, and not the limits of the aircraft. In the spirit of testing limits, what you can demonstrate becomes the limit, even if nothing wrong is noted. Since ILS approaches are seldom steeper than 3 degrees, the limits become 3 degrees. Sort of a circular argument, chicken and egg wise.
In carefully controlled DGPS tests we were able to take an S-76 regularly to 6 degrees of descent when we started at 80 knots, and up to 9 degrees if we started at 70 knots and decelerated to 30 knots over the threshold.
The steepness of the approach is determined by the ground speed during the approach. Logically, if you have zero knots forward speed, you can approach at 90 degrees, can't you? In fact, most VFR approaches end up at about 30 degree descent angles near touchdown (90 degrees in the last 2 feet!)
Nick