The combo is variable depending on your intention. MPA can add the physical stimulation to the almost entirely cerebral 3-FPM experience and so can be beneficial for productivity. Adding MPA to a 3-FPM session removes much of the euphoric aspects though.
Mixing substances that cross similar neurophysiological pathways, or possess the same mode of action, is bad. You loose horsepower.
Something like a dilution. Imagine that you want paint black your wall, and someone puts grey in your paint.
Finally the wall will be dark coloured but will not be black. Just grey-black.