With the first fall of good snow in New England each year the advance guard of skiers dotting the white landscape is sure to include the five Buchmayrs: Sig Buchmayr, one of the area's best skiers, his wife Sally and their 15-year-old triplets Sig Jr., Norbert and Charles. Austrian-born Sig established one of the first ski schools in the U.S. when he began teaching in Franconia, N.H. 28 years ago. Now residents of Manchester, Vt., the Buchmayrs' busy ski life includes operating ski shops in Big Bromley, Lake Placid, Yonkers and New York City. The triplets, who attend Burr and Burton Seminary in Manchester, took their first ski lessons from their mother when they were four, then progressed to training by their father. Now expert athletes in their age group, they are collecting victories in ski events. But high time in the hills is still when the five Buchmayrs swoop by in an intrafamily downhill race.