How do you create planar surfaces

Tim,

You cannot create a circular or triangular "surface". You can however, create a circular or triangular face that uses an underlying planar surface. To do this, create edges defining your shape (a single circular edge or 3 linear edges creating a triangle) usually using BRepBuilderAPI_MakeEdge. Then, create a closed wire from the edges. Finally, create a face from the wire. Since your face uses a planar surface, the easiest constructor would be:

BRepBuilderAPI_MakeFace
(const TopoDS_Wire& W,
const Standard_Boolean OnlyPlane = Standard_False);

Pass your wire and Standard_True for OnlyPlane.

For example, to create a circular face:
gp_Circ circ(gp::XOY(), 10);
TopoDS_Edge edge = BRepBuilderAPI_MakeEdge(circ);
TopoDS_Wire wire = BRepBuilderAPI_MakeWire(edge);
TopoDS_Face face = BRepBuilderAPI_MakeFace(wire, Standard_True);