Four years ago, Pittsburgh’s Three Rivers Arts Festival decided that it needed more than just art to draw in the public, and launched a carefully tailored music component designed to dovetail with its existing visual art base. So far, it’s been a winning scenario for everyone involved: since the performances, which take place in the midst of the art fair, are free, crowds are far bigger than they would be at a stand-alone show with an admission charge; and the guest performers bring in people who otherwise might not think to attend an art fest.