The case is being investigated by the Mutual UFO Network (MUFON), a US-based organisation dedicated to UFO and alien research.
It also keeps an online database of UFO sightings, to which witnesses can send accounts pictures and videos.
The witness was walking through fields at 7.20am on January 16 “assessing tide flats for duck hunting” when the incident occurred.
The event was reported to MUFON this week.
In a report, the unnamed witness said: “I observed three white, blinking lights in triangle formation to the south.
“I presumed them to be aircraft, but was not 100 per cent sure.
“I expected them to break formation, but they seemed to stay in formation longer than commercial aircraft would.”
The witness took photos and two short video clips and submitted his report to MUFON on February 20.
Washington MUFON state section director Tim Ward is now investigating the case.
Roger Marsh, MUFON communications director, said: “Please remember that most UFO sightings can be explained as something natural or man-made.”
Scott Brando, runs hoax-busting website ufoofinterest.org, and forensically investigates alleged UFO sightings.
He said planes circling and waiting to land can sometimes appear to have made a formation.