WATERVILLE — Colby College plans to start building three athletic fields this fall on about 19 acres behind the Harold Alfond Athletic Center, with a goal of having them completed and ready for use in the fall of 2017. There will be an artificial turf field for soccer and lacrosse, a grass field for soccer and a third practice field to be used for soccer and other sports, according to Mina Amundsen, Colby’s assistant vice president for facilities and campus planning.