the acid strength of an acid depends on the number of replacable hydroens it gives per mole of the acid

from the structure of the above acids (see below) we can say that H3PO2 gives only one replacable hydrogen while the other two dont give replacable hydrogens(as they are enclosed in a strong bond)

souce :encyclopaedia brittanica and http://wps.prenhall.com/wps/media/objects/602/616516/Media_Assets/Chapter19/Text_Images/FG19_10-06UN.JPG