Cool build! Good to see a fellow Kansan. Where abouts are you located? My wife and I just bought an older home south of Salina and are in the process of "rehabbing" it. Looking forward to seeing what you have in store for your new building!
6/3 SO will only give you 120v loads. 1 hot, 1 neutral, 1 ground. Or you can only run 240v loads; hot, hot, grd.
oldwizard is correct, 12/2 nm (romex) is 2 conductor with ground)
hope this helps.