Interesting one this. I wasn't anywhere near Phoenix obviously - I'm over in Somerset - but I've been cross-referencing this against some data I pulled from my Unistellar eVscope that same night. Running it alongside FlightAware logs is something I do routinely now after a cluster of sightings we had near Glastonbury last autumn.
The triangular formation specifically - was it solid craft or three distinct light points holding formation? That distinction matters enormously. What you're describing sounds consistent with a handful of incidents I've catalogued where the craft appears solid but thermal imaging (I run a FLIR Scout TK alongside the scope) shows no heat signature whatsoever along the body. That rules out conventional propulsion pretty decisively.
My actual question for those who witnessed it: did anyone notice any temporal anomalies during or immediately after the sighting? Watches running wrong, phone timestamps glitching, that sort of thing? I've been building a dataset on this correlation for about three years now and the triangular UAP cases disproportionately flag these effects compared to disc or cylinder-type encounters. Could be electromagnetic interference, could be something stranger involving localised spacetime distortion - I'm genuinely not sure yet.
Also worth asking: what was the approximate altitude and apparent speed? The Phoenix area has previous form for exactly this type of report and I'd love to map this against the 1997 Phoenix Lights trajectory data I've been sitting on.