An installation is using a BT broadband connection on WAN1 and a K3520 dongle on WAN3. The dongle gets a good signal (the 3G mast being just 100 metres away).
Setup is for Load Balance according to Line speed but users are complaining of very slow speeds unless I take them out of the equation by putting them in a Load Balance/Route Policy to only use WAN1.
Any ideas as to how I can get this setup to work using Load balancing?

Check to see what speeds you get with the dongle when WAN1 is disconnected and no one is using the connection. If the speeds seem reasonable then try setting appropriate DownLink and UpLink speeds in the WAN General Setup page for WAN3 to see if that balances more accurately.