I think you save the 6' of 12-3 anyway; you only need 12-2 between the switch and the light. It looks like you might save a little 12-2 by running the light at the top of the picture from the first light, rather than putting it at the end of the chain. That would result in a smaller voltage drop as well at that 3rd light, but I doubt you'd notice (the difference is about .3 volt). As drawn, the voltage drop at the far light (assuming all lights are working and the far switch is controlling) is under 1 volt, well within the 3% guideline. Copper's cheap these days; I'd go with whatever's easiest to install.