The formula to get an endpoint latitude and longitude (lat 2,lon2) (in radians) from a starting location (lat1,lon1) (also in radians), range (d), radius of the Earth (R) and bearing (brng) is:
var lat2 = Math.asin( Math.sin(lat1)*Math.cos(d/R) + Math.cos(lat1)*Math.sin(d/R)*Math.cos(brng) );
var lon2 = lon1 + Math.atan2(Math.sin(brng)*Math.sin(d/R)*Math.cos(lat1), Math.cos(d/R)-Math.sin(lat1)*Math.sin(lat2));
Your start point will be one of the points of the triangle polygon. The other two can be found using this code snippet, putting in your bearing +/- 15 degrees (for the 30 degree triangle example) and a range that depends entirely on how big you want the triangle.