The postmodern literature is characterized of showing the truth as a perspective of each one, in other words there is no "the true" but different realities. It tries to find a middle point and the interest of all the people so everyone can feel confortable to read it.
With this explained, we can assume that "A 3 act play about a wedding" can be classified as a Postmodern piece of lterature because each act can be tell from the perspective of 3 different characters.