I think Antiphon is right about the actual reason why it's a triangle, out of convenience, but also it's a shorter distance between the three sides of a triangle than a square or a circle, using the same amount of material, so you can strike it faster.
but the sound would be a different tone for a circle than it would a triangle. I think the sharp angles in triangles effectively make it three different lengths of metal as far as fundamental harmonics is concerned (depending on the material and how it creased).
A circular triangle would be like a longer string, effectively emitting one deep tone instead of three higher tones.