Integrate several javascript adfeed APIs into JSONP script

We need to integrate several javascript ad feeds into a single javascript/jsonp ad feed. We have several seperate javascript based APIs that call for banner ads from ad networks. When there is no ad available, these scripts return an error. We need to create a single script that can call several of these api's in a pre-defined order, so if one network doesn't have an ad, another one will be called. This script will feed a jsonp based script that displays ads on our mobile website. In addition, we'd like to be able to easily switch out which scripts run first, second, third. We will supply additional info on the job upon request.