Triangles are usually better than "T"s. What kind of bracing can you add at the right angle joins? Evem better.can you design it as a triangle support or a rectangular support?

Oh wait OK let me totally rephrase this. Under a bending stress which can sustain more stress an equilateral triangle, a square, or a circle assuming the height of the triangle, side length of the square and the diameter of the circle are all the same value. I think this is what I should have been asking from the beginning

Just a weight placed right in the middle like if there's a tube, a box beam and a hollow triangular structure each a meter long with the same wall thickness, held at either end and each with a force pressing down on the center. Which could withstand the most force?

I'd be very reluctant to build anything from PVC if I wanted it to last a long time. PVC is a plastic type material and deforms plastically with the passage of time. Plywood has some of the same problems, but to a much lesser degree in most situations.