After this thread has played out a bit, it is starting to appear that you want, in addition to 3-plow capability, a LOW PRICED tractor with a certain amount of collectability. With that, I'd look at Allis Chalmers, Case, Oliver, IH, and some of the more obscure Massey's (ie MF85, 88, 90) There's where your "cheap horsepower will be found these days. Brands that are still "current" typically have a higher value even as they get older.
And a question that has been left unanswered is the issue of draft control. If you'll be using a mounted plow, draft control is VERY worthwhile in any soil conditions. If you're looking at a 4 or 5 plow rated tractor to pull a 3-bottom plow, draft control probably isn't as important though. A bigger tractor with a (relatively) smaller plow can just "muscle" the plow in most any conditions. And it's been my findings that the more adverse the soil conditions are, the more valuable draft control can be to maintain constant working depth.