3 dark objects, size undetermined, in formation at extreme speed

dates unknown, late 1970's, summertime. 3 teenagers (1 male/2 female):  my brother and my friend and I were lying out in the backyard, watching for satellites.  
Suddenly 3 objects flying in formation crossed over us.  Because it was dark out, and these objects were also dark, it's impossible to say how big they were or how high they were.  
They were each identical in size, oblong, what reminded me at the time of the shape of an over-sized rear-view mirror my father had installed on the inside of his car: long and slightly rounded at the edges.
Witness indicates that the date of the sighting is approximate.  PD