The firest a subject thst is a triangle itself.
For this image I have chosen a rack of coathangers. They are clearly triangles.
The second image is a triangle by perspective, converging upwards. I have used a wide angled lens taking the shot on the corner of the building looking upwards. A classic image for this triangle.
The third image is an inverted triangle converging downwards. A clump of snowdrops gave me my nice tidy triangle. In this image by placing the clump of flowers to the left of the frame, the leaves and ivy in the right of the frame also form a triangle, with the point of the triangle meeting the same point as the triangle formed by the flowers.