It looks like it requires three separate notes. I can't tell you whether the acciaccatura is played before the beat or on, that depends on context/period (i.e. classical vs. romantic vs. baroque eras). But one way to de-emphasize the third note is to play it with your second finger - and that also sets you up for the rest of the phrase. It looks like it should be played with the thumb but that might tempt you to accent it.