triangle UFO seen by 3 people

In 1993 or 1994, we (me, my brother and my mother) saw a triangle shape light in the sky, it was a bit cloudy, so part of the object was blurry as clouds were blocking it, but we could definitely distinguish the shape in the sky.
The UFO was staying in the sky, it didn't move.  It was about 1000-2000 feet from the ground.  We were staring at it for about 5 seconds, when it suddenly took off in a massive speed.   It didn't rise up in the sky, it started flying like an airplane at the same altitude where it was staying before.  It took of instantly like in a sci-fi movie, it flew right over our heads and we could hear the breaking of the sound barrier for about 5 seconds or so.  Massive roar.
I think IT knew that we were staring at it, coz we were pointing at it and saying, "Whoa what's that?" when it suddenly took off.
It might have been a secret military plane, I don't know, but it really moved so fast, like it was a science fiction movie.
The size if the triangle could have been somewhat similar to a Boeing 757 or so..