Triangle craft heading SE to NW across evening sky

I was in my backyard and was looking at the night sky as I often do and almost in my exact line of sight I witnessed a perfect triangle craft with 3 dim white lights on each point move silently across the horizon from SE to NW from my location. My guess it was about 1500 feet off the deck and moving around 500 miles an hour. I never saw it change direction or speed but the fact it was so quiet at that speed was eerie. I saw it for about 3.5 secs before it was no longer visible but I couldn't have seen it any more clear and couldn't have any less doubts about its shape.