From an electrician's point of view I have a couple of suggestions. You need to fuse each load with fast acting fuses. The reason for this is your main battery. I am going to assume that it is a motorcycle battery. If there is a problem with one of your devices a lead/acid battery will start to pump out more amps then your devices can handle. Perhaps a main fuse is in order to limit the output of the main battery.
You have multiple voltage meters and I am not sure why. The one connected to the DC buss would give you an indication of system voltage, the others?
The last thing is the 3 way switch. Generally speaking a 3 way works as follows; position 1, all circuits off; position 2 circuit 1 is on and circuit 2 is off; in position 3 circuit 2 is on and circuit 1 is off, if that is the way you want it to work. If you had something else in mind you need a switch with a special cam set up which would increase the cost. Hope this helps, WJ.
Thank you for the reply! Great note on the fuses, I have been looking at a few fused power distribution blocks. I will make sure to drop one in line. (after the 2 position switches?) I must have been too dug in and missed the redundancy with the volt meter, you're right one should be good. I have been re thinking the idea with the PC Power Supply, I might make it a stand alone system in the case now that I have isolated the 12v and 5v from the Power Supply. I also kept the 3v just in case there is a need for that later.
But thank you so much for your help!