The redrawn diagram should look like this:
Original diagram
Corrected Diagram
The multiplicity of the Triangle end of the association should be zero or more (0..*) or one or more (1..*).
Use * if you will allow your system to include points that are not part of a triangle.
Use 1..* if you record only points that are part of a triangle.
The Point end of the association must have a constraint that does not allow triangles to be constructed using three points that are collinear. This will prevent test cases three and four from occurring.