I thought that the answer may be in a combination of TERPS and FAR-25. However, there appears to be a mismatch in the requirements for twin / three engine aircraft.
From TERPS:- No obstacle may penetrate an obstacle identification surface sloping with a 40 : 1 gradient (2.5 %). When climbing from MDA with the required gradient the aircraft shall clear all obstacles with the prescribed margins. (94 -164 ft ?)
PANS-OPS has a note:- When obstacles in the missed approach area determine OCA/H, a steeper gradient gives a lower OCA/H. This steeper gradient must be stated in the instrument approach chart. i.e. the operator is responsible for the a/c performance
FAR-25 -121 (d) Approach. In the approach configuration corresponding to the normal all-engines-operating procedure in which Vs for this configuration does not exceed 110 percent of the Vs for the related landing configuration, the steady gradient of climb may not be less than 2.1 percent for two-engine airplanes, 2.4 percent for three-engine airplanes, and 2.7 percent for four-engine airplanes, with--
(1) The critical engine inoperative, the remaining engines at the go-around power or thrust setting;
(2) The maximum landing weight; and
(3) A climb speed established in connection with normal landing procedures, but not exceeding 1.5 VS.