Click on all 3 vertices, then back the the first point of the triangle to close it.
Sprinklers
Draw a triangle connecting all 3 vertices.
Construct the perpendicular bisectors if you want the circumcenter to put a circle outside the triangle. Construct the angle bisectors if you want the incenter to put a circle inside the triangle.
Perpendicular bisector, click on the sides.[br]Angle bisector, click on 3 vertices with the angle you want bisected in the center.
Construct the circle.
Click on the center first, then click on the where the bisector intersects the vertex or side of the triangle.