Summer situations are over just too soon and these three long short stories are foreclosed with that certain sense of loss which comes with the first fall day. They are prepossessing and faultlessly overheard versions of young and young married experience. The least of these is the first -- ""Love in the Dunes"" -- in which half of two couples who take a house on Cape Cod together go on to share each other. More substantial and progressively shriller is ""How I Spent My Summer Vacation"" in which a child of divorce visiting his father and his stepmother makes a treacherous phone call to his mother back home, curtails his stay to ten days and leaves them both to settle up rancorous feelings about everything with each other. The third has a Paris setting in which the relationships of two Americans abroad, both students, is rudely disrupted by the arrival of a former teacher -- a ghastly European refugee intellectual who ""materializes like the damnation of Faust."" Small pleasures to be sure but Miss Birstein is an astute and altogether attractive writer.