Where do they come from? These young nymphs piling into Serendipity at 9 p.m. for mountainous ice cream sundaes. Their slim forms are just one of the many wonders of Serendipity 3, the eccentric ice cream paradise at 225 East 60 Street. Literally one block from Bloomingdale’s size 2 racks, Serendipity has long attracted the fashionable and the famous, but above all, just folks for 65 years. How many places last this long without giving up their groovy 1960s font?