Squaring up to each other

I was always the biggest of the three of us and that's the way it would stay. We are a family of three and you know what they say, two's company, three's a crowd. However, like all families ought to be, we are united, straight and true, each dependent upon the other two, but always taking sides opposing the others. My brothers always said their angle was right, but it didn't matter to me. I would always be the biggest.

When squared up to each other, I was the equal of both of them put together. Pythagoras had proved it.