Has anyone used these Trigrip reflectors?
Lastolite TriGrip Reflector, Sunfire/Silver - 30" LL LR3636

I'm wondering if the shape is more stable in the wind than a circular one.
California Sunbounce Sun-Mover (Zebra/White) C-SM8-820 B&H Photo

I have a smaller version, but haven't used it as a butterfly or on an arm yet. It's doubtful I will. How are you mounting your circular reflectors? I've had mine in breezy conditions (5-10mph I'm guessing), without issues. It helps to have a reflector arm and solid c-stand well ballasted. I'm guessing that surface area plays more of a factor than shape in wind buffetting. However, all else being equal, I'd rather be a circle in the wind than a triangle.

Grips hold them. Regardless if human or stand, if you think about holding a reflector in two places along the edge, a circle results in two 180 degree semi circles exposed to the wind. A triangle would seem to present a much different and perhaps more rigid shape.