I just added a third WAN connection into my pfSensebox for LAN parties. However the configuration was pretty tricky and I forgot most of the settings, so I wrote this quick guide to follow for the future (if I have to do it again..)
The fist step is to configure WANs. Just configure DHCP and internetsettings separably to each WAN.
Go to System->Routing->Gateway Groups, and add new
go to Settings->Routing->Gateways and select a gateway to edit.
The Weight should be configured in the way that the best connection has most weight (more traffic goes into that one). I guess that pfsense is using the weighted round robin algoritm for routing. you can see on the wikipedia page how the weight should affect the traffic routing https://en.wikipedia.org/wiki/Weighted_round_robin
The Gateway group can be taken into use in Firewall settings. Go into Firewall->Rules->LAN
The trafic shaper is very easy to configure with multilan, thanks to the new traffic shaper wizard (under firewall/traffic shaper). It basically contains settings for prioritizing games, and detecting game downloads (steam etc), so we can put downloads into lower priority than games.