While out in my backyard with my dogs at above time I hear the unmistakeable 'jet-like' roar.. EXTREMELY loud sound..of craft that have passed my skies in previous years, as reported by me herein. This engine 'roar' is unlike conventional jets, military or otherwise. The sound was so loud as they approached from the SE (which would be from the Atlantic ocean side) that it seemed they would come right over my house by a few hundred feet...not thousands of feet, as they did. I looked up to see three, black, triangular craft racing toward the NW at about 15,000 feet. The three craft were about the size of my pinkie finger nail at arm's reach. The amazing thing about these 3 craft, other than their deep, bass, loudness, was the fact that they were perfectly spaced from each other across..not forward to back. The 3 were identical in shape and color..no after-burner or smoke of any kind...but perfectly spaced, both horizontally and vertically from each other, by, perhaps a mile or so. Only the Navy's Blue Angels could come close to this juxtaposition, I believe. The sound was the same as in past years as was the direction of travel..SE to NW. Their first appearance, as I've reported previously, was very dramatic at rooftop level flight, and doing acrobatic maneuvers not consistent with conventional jets..their lighting was totally different as well. Since that first encounter, about this time of year 3 or maybe 4 years ago, their appearances have been at much higher altitudes and their plane of travel has been perfectly straight and level. I saw no lights from these craft today, but have in the past, at night, as one simple large blinking red light underneath each one. My first sighting of these craft, as mentioned, involved multi lights on each craft, going from all white to all red and back to all white..very strange. I don't know what these craft are but they are NOT conventional jet military aircraft. This is continuing and very odd. This is the fourth encounter of these craft here over the past four years.