garden network and power cables
You will want to limit the voltage drop in your DC distribution wiring to 5% or less of the 12V. That would be equivalent to a 0.6V drop. An online wire size calculator suggests that for 600 feet at 3A that you would need to use 1 AWG copper wire. 

Cost of that size wire suggests that you may want to consider distributing the AC mains voltage instead and converting to 12VDC at the load points. The reason being is that the amount of voltage drop in the wiring follows the amount of current through the wire. 30-40W at a line level voltage results in less than 0.5A down the wire and 5% voltage drop at 120VACrms makes room for a larger allowable voltage drop than the 0.6V at 12V. The net result is that a much more reasonable wire size can be utilized.

If you use optical fiber cables they could be in the same conduit as DC cables. You may want to separate them from AC mains cabling just because it seems like a good idea to keep AC mains by itself.