Obviously not because of gravity or to make it stable enough in wind, but then why?
I imagine that the [diffraction spike pattern](https://en.wikipedia.org/wiki/Diffraction_spike#/media/File:Comparison_strut_diffraction_spikes.svg) caused by a single supporting element would be preferable to that caused by 3 of them, so there must be a good reason.
Edit: perhaps more important than the diffraction spike pattern, a single supporting element would mean less weight we need to get into space, which is expensive.
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It’s a moveable mirror, having three arms is a really simple way to give it a ton of range of motion without really complicated mechanisms. Like you can tilt it to any angle in it’s range with grade school trigonometry and a simple motor, instead of having it on a single complex robotic arm with a lot of digital room for error.
> Edit: perhaps more important than the diffraction spike pattern, a single supporting element would mean less weight we need to get into space, which is expensive.
The opposite would be true. It’s like deciding whether to use a truss or a single beam to build a bridge. A single beam with the same strength will weigh more than a truss.
While there isn’t wind, there are vibrations due to thermal expansion, thrust events, etc and the three piece construction may be much more stable.
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