After several wrong turns, I've finally gotten my rig to play nice with three-(3) WAN connections load balanced.
Everything is running on an old Intel P3-1000 with 1-GB of RAM and 40-GB PATA drive. Obviously its got four-(4) Ethernet ports with parts scrounged from various spare cases in the garage.
How'd I do it? More details on the adventure shortly....
There's a fairly easy Multi-WAN tutorial to follow over at pfSenseDocs which deals with a basic dual-LAN setup. Using the funny discussion at one of the forums gave me confirmation of what I needed to do. Next thing you'll need to do is make sure outbound is likewise working.