Triangular have been seen since the 1940's but have increased rapidly since 1990's. Many skeptics of UFO's go to possible government files of them working on flying disk technology. There are also records of triangular UFO's but, in August 2004 the organization (NIDS) found that sightings did not conform to previous deployment of black project aircraft and that the objects' origins and agendas were unknown. So it's hard to explain the triangular UFO's. Are the triangular UFO's really from other planets?
One of the most famous UFO sightings was the Phoenix Lights incident. There was multiple UFO sightings some as low as 1,000 feet and moved far too slowly and silently for a airplane or helicopter. Also there was a V shape to the objects. Another thing was the speed. Just like in other UFO's, they accelerated too fast and slowly to be man made. The big lights pointing down on the city could be seen from as far away as Las Vegas some reported, but seen much better in the city. An official report made by the Air Force about the incident concluded that the military had been testing flares launched from conventional aircraft during that time. The Air Force even tried to recreate the event, going in their V shape formation but the noise was far to loud and didn't recreate the event. Also the object was reported to be a mile long, longer than our aircrafts.
Since it can't be explained, they need to be studied more.