I observed three triangular luminescent objects in delta formation pass silently overhead very quickly.

I'm a musician.  On August 25, 1990, I was playing a gig in Townsend MT outdoors behind the Mint Bar.  I know the date because I have records of all my gigs, and I played at this place only once.
During a break, I stepped into a gap between the Mint and the building to the north.  I was facing north.  Within the narrow space, there was nowhere to look but up.
I saw three luminescent, triangular objects in delta formation fly very quickly from east to west; they spanned that stretch in less than three seconds.  I heard no immediate or delayed sound. A rough estimate of altitude is 10,000 feet above the surface.
A few years later, I was playing music in Europe with a couple musicians from Montana.  One of them was living in Great Falls MT in 1990.  I talked about the experience, telling him only that I had seen three luminescent objects.
He asked, "In delta formation? Traveling fast?"  I said yes.  He said (although we had no means to compare dates), "I saw them too.  We had stopped near Eddie's Corner, and I saw them fly overhead - very fast."  Although this happened 27 years ago, I recall it vividly.  I hadn't thought of it for quite some time until recently reading "Montana UFOs" by Joan Bird (2013).
((
We spoke via telephone with the source of the report, and he sounded to us to be