I dont think you could keep a corn (Best started snake) in a 3 foot viv. I would say 4 foot would be fine. Problem is that they grow to up to 6 foot, so it will be fine when it is young, but when it gets older, it will be cramped. I think the 3 foot viv would be perfect for a royal!
But as mentioned they do have problems with feeding, but if you get a good feeding one, that you know has had a feed (If you get it from a reptile shop, ask to watch it get fed) and if it does feed, you have hopefully got a good one there.
If you have any queries, just ask!