Three black objects seen in early evening sky

It was just before dusk and my mother noticed 3 black objects in the sky.  At first she thought they were birds when she realized they weren't she called me to come take a look. We watched them for about ten minutes until they were out of sight.  
There was one in the front that appeared to be higher then the other two.  Two of them were traveling at the same height and were in a straight line with one in the front and the other one behind it.  
They seem to stay the same distance apart for the entire time we saw them.  They were traveling northwest.  
The objects made no sound and had no lights on them.  We live in the South Hills of Allegheny county outside Pittsburgh.  I was able to get a picture of the three objects.