The 3 rivers all meet in the city of Pittsburgh, Pennsylvania. Those three rivers are; the Ohio river, the Monongahela river and the Allegheny river.
1 Additional Answer
The three rivers that meet in Pittsburgh, PA are the Allegheny, Monongahela, and Ohio Rivers. The unique joining of these river systems is diverse and very abundant. The Monongahela is the oldest of the three river systems. You can find more information here: http://www.pittsburghgreenstory.org/html/3_rivers.html