3-hour layover at STL? I'd hop onto the light rail line (METROLINK....it's quite cheap) and cruise over to downtown St Louis. The trip time is a little over 30 minutes per direction and service frequencies are adequate 7 days a week (though, after 10pm, headways are approximately 30 minutes with service ending aroud 1am).

Downtown STL has a few things to see and do. Mississippi River, Laclede's Landing (bars/pubs), Gateway Arch, etc. Downtown has improved a whole lot over the past decade (particularly at night since most of the streets are now well-lighted). I'm quite impressed.

Cheers,
MIKE