And this is....

[WitchAndShaman]

Many of the hobbiest drones, especially the quad-copters, use electric motors that are almost silent. Those I've seen emitted just a very soft whirring sound, and were audibly discernable only when they were very close. Add a couple lightweight LED lights to a quiet craft like that and you've got an instant UFO.

Also, because three points always lie in the same plane, this would be an easy form to hoax. Heck, you could even be lazy with some minor unintended vertical position change by just one drone and the triangle remains still roughly intact.