sounds like just a variation on navigation lighting. When I began investigating UFO sightings I quickly learned that the flash patterns and placement
of the lights on aircraft actually vary widely. This came up as a result of a sighting in which the plane was using a quad flash arrangement I had
never seen despite growing up on an air force base.
The new flash patterns are usually strobes known as the "comet" which use multiple flashes to increase visual "on" time. These are replacements
for lights which were formally rotating beacon types.
The 3 flash pattern is a well known helicopter pattern, btw.

