IT was a WASP love triangle at the New Yorkers for Children Fall Gala at Cipriani 42 on Tuesday night when Topper Mortimer and his girlfriend, Vogue editor Valerie Boster, ran into Topper’s ex, Tinsley. “Val wasn’t happy, but everything was OK by the time they went to their respective tables for dinner,” says a spy, adding, “Topper is more ‘normal’ now that he’s dating a normal girl.” Tinsley showed up with her own squeeze, Prince Casimir Sayn-Wittgenstein-Sayn, at the opening of the Douglas Hannant store in the Plaza last week, where a spy reports, “He stood around in the background holding her tote bag.”