Hi
Somthing like this should do it:
IPointCollection pPoly = new PolygonClass();
object Missing = Type.Missing;
pPoly.AddPoint(P1, ref Missing, ref Missing);
pPoly.AddPoint(P2, ref Missing, ref Missing);
pPoly.AddPoint(P3, ref Missing, ref Missing);
((IPolygon)pPoly).Close();
Have Fun
Mody