In the OP the ufoevidence.org website is cited, so it seems to be acceptable to the moderators and others on this thread. So I ask to be forgiven for citing the same source in reference to the famous Belgium flap of '89 and '90. This is by far the best investigated case involving our topic, Flying Triangles.
"...the two photos supports statements by the pilots that the UFO dove from 2,000 meters (7,000 ft) to 00, indicating that it was below the 200 meter limit on the radar. This occured in ONE second."
The F-16 pilots plus up to (4) radar sets have the UFOs performing sharp turns and accelerations, including diving either into the ground or stopping short in a ridiculously brief amount of time and space before ascending again. On the surface, this might be evidence that gravity was being manipulated, or more likely that the object simply lacked much in the way of mass and consisted mainly of energetically ionized particles which might change speed, altitude and direction in response to radar or other stimuli.
I now cite Ivan Seeking. Well researched was he on the subject of UFOs and radar.
In brief, I think that although Flying Triangles pose a slightly more difficult problem than balls of light, the physics involved are similar enough that the prosaic explanations of unusual meteorological occurrences and electrical eccentricities are greatly to be preferred over extraterrestrial, i.e., alien
hypotheses. The going gets stickier when assertions are made, like by the Belgians, that the objects exhibited intelligent behavior. But these claims are countered by experimentally known adverse effects of electromagnetic fields upon human consciousness, and too by the innate desire for exotic solutions driven by the wishful thinking and imagination of overawed humans.