Hi Johannes,
what a novel approach to naval architecture!:-)) Afraid your idea of a third engine is out of the question for the foll. reasons:
1) Hull too narrow to put it side by side with the others, thus you will practically have to double the length of the E/R.
2) Aside from the obvious inefficiencies of a triple screw propulsion, where would you place the third prop?
3) An extra 2 knots still will not be fast enough to outrun the escorts and planes, which as of '43 were pretty efficient to force the boats under water and kill them.
4) If the couple knots were realy mandatory, the easiest and most cost-efficient solution would be to add on 2 or 3 cylinders to each engine, lengthen the DieselE/R by 1/3 (less than 2 meters).
Rgds, RB