When both headlights go out it's the switch or the wiring to the switch. Each element of each headlight has it's own fuse. The battery connects directly (almost) to the headlight switch and the switch to the buss for the four fuses.
Which fuse 3? the one on the aux 9 fuse buss or the one on the 15 fuse buss?
1 through 4 on the 15 are the headlights.
3 on the aux 9 is one of the two cooling fans on the radiator. Most likely the older 3 bladed unit with a frozen bearing. This does need to be replaced or you will kill the engine from over heating.
Fuse 4 on the aux 9 is for rear window defogger. If it's been reconnected to 3 then there are some serious problems with the wiring.
http://www.clarks-garage.com/shop-manual/repair-procedure-index3.htm#fuses
http://www.clarks-garage.com/pdf-manual/elect-10.pdf