ONE relay only and no diodes.
Connect the relay POWER cables as usual, IN from a fused power connection to +ve, and OUT to the lamps.
Connect the solenoid earth to deck firmly.
This leaves the one tag that if it goes live (+ve) switches the relay on, and your foglamps will light up.
Connect that tag to the CENTRE tag of your ON-OFF-ON switch.
Now then, take a cable from the top tag of the switch and scotch-lock it to the MAIN BEAM wire near a headlamp. Get the idea?
Now take a cable from any live source (that goes OFF when you turn off the IGN key) or even better scotchlock it from a SIDELIGHT wire and connect it to the lower tag of the switch.
I bet you can work it out now!
Anyway, driving along in the dark, switch in middle position, with the headlamps on main beam - switch UP, and mainbeam power trips the relay and foglights come on. DIP mainbeam, and headlights dip AND the foglights go out too.
Driving along in day or night sidelights at least on, switch DOWN, and the foglights come on and stay on till you turn them off.
Best of all, switch in middle position disables foglights.
Sorted.
CharlesY