Not my area usually - I'm mostly buried in EVP work down in Cardiff - but this caught my attention because a mate forwarded me some shaky phone footage that supposedly matches what you're describing.
Quick question before I dig deeper into it: was the formation static or drifting? The clip I was sent looks like a slow westward drift, maybe 2-3 degrees per minute if I had to estimate, but the quality is genuinely awful so I can't be certain.
Reason I ask is that the behaviour changes everything here. Static triangular formations over populated areas tend to have pretty mundane explanations - coordinated drones are the obvious one, and Phoenix has had organised light shows before. Drifting formations are a bit more interesting to document properly.
Also wondering if anyone had any audio recording running at the time. I know that sounds like my EVP bias creeping in, but unusual aerial phenomena occasionally produce infrasound or RF interference that standard phone mics can actually pick up in the low register if you know what to look for in the waveform afterwards. Audacity would show it clearly enough.
What kit were people using to capture it? Phone cameras, dedicated camcorders? Anyone run anything with manual exposure settings? The metadata from a proper camera file would at least nail down the timestamp and exposure duration, which helps rule out long-exposure artefacts.
