The tertiary sector of the economy is the service sector, the sector that produces services and not goods.
One example of an activity from that sector is a law firm. For a law firm, there is very little "land" involved in their work. The only "land" involved is the property on which their office building stands. Their physical capital will consist of things like their computers and copy machines. Their human capital consists of their staff and the education that their staff has had to get in order to be qualified. The labor, of course, is the work done by the staff every day.