I think SUX at one time had mainline jet service on three airlines--UA to DEN (727s and 737s), HP to PHX (737s), and TW to STL (DC-9s). Plus turboprop service on UA express to ORD and NW Airlink to MSP. (NW didn't start DC-9 service until after UA pulled out.)
LNK, close to my former home town in Nebraska, used to have mainline service on four airlines--UA, TW, HP, and CO. Now it's all regional jets, and no CO or HP anymore.