No man, if you are running your dryer or stove off of 12/3 be on the look out for a fire, in NO case should either of those appliances be powered with 12 ga wire. That's bad. But what you can do is look in your code book that you definitely have if your doing electrical work right?...lol in there and after cross referencing 2 charts and examining the specifics in the code that may pertain to your room (I.e., bathroom, kitchen, garage, etc. ) then you can determine the proper size cable for your application. Wire size is way more about amperage than voltage, but like growurowndank said, im scared to say too much. I wouldnt want anyone talking my advice and killing themselves. I'd be scared for anyone to be doing work without already knowing this info. Try mikeholt.com for electrical forums and info, but they'll say the same thing, except way harsher, higher an effing contractor. JSB, thanks for the "like" on my post before, but I'm sorry, please have your work inspected by a licensed person. Your missing some key understanding of electricity.
Peace.