Three triangles in formation moving fast and silent.

Whilst outside in my garden I was looking at the stars and a meteor caught my eye, so I decided to look in that direction for more. After around a minute I noticed 3 dark triangles in formation, 1 in front and two to the rear travelling at speed from the North East direction headed Southwest.
The speed was amazing and no with no sound they travelled most of the visible sky in around 5 seconds, I live below a flight path where planes fly at cruising altitude and they can be heard.
I estimate them to be at a quite low altitude around a few hundred feet as they seem to be lit from street lights which made them harder to see.
None of the them had lights.