I don't know what the word is, but I do know that there is a word for how a triangle is not going to change shape and is so stable. This phenomenon is at the heart of the reason that so many triangles are used in engineering--bridge building--other construction. Put together a triangle of three Popsicle sticks into a triangle. Unless you break it, it will stay the same shape.
It's what makes a three legged stool sit stable where a four legged stool wobble if one leg is even a few millimeters longer or shorter than the others.
Is there a theological significance in this solidity--stability of three?
A square or a rectangle is not like this. If you push on the corner of your Popsicle stick square, it will turn into a rhombus. The angles change easily. A rectangle turns into a parallelogram.
So here, from my walk the other day, is what happens to a square in real life.