Hi,
You can add a triangle shape only if you make a widget that has triangle as a ShapeType. The default "Shape" widget that you can find in your Thingworx Composer only has Circle, Ellipse, Rectangle and Rounded Rectangle as shape types. In your case you need a triangle shape type, for this you need to create a new widget that is similar to the "Shape" widget and just add another type(ShapeType).
Thank you,
Adrian
