It is a triangle. The abstraction of great lines in hyperbolic geometry are geodesics and just like three lines form a triangle, so do the geodesics. If you walked along the earth’s surface from the equator to the north pole and back, taking 90 degrees angles every time, you will have felt that you made a triangle by walking straight in three directions.

The reason why the angle sum can be more than 180 degrees is that the sphere has a positve curvature. If you want one with negative curvature and less than 180 degrees angle sum, try to make one on the side of the hole on a torus (look up its curvature if my explanation was lacking).