Found:

ABX discrimination

PHONETICS: Of the three stimuli presented on an ABX trial, the third is the focus. The first two stimuli (A and B) are standard, S1 and S2 in a randomly chosen order, and the subjects’ task is to choose which of the two is matched by the final stimulus (X).

LIT.	Macmillan, N.A., & Creelman, C.D. (1991)