I bought my corn snakes from my herpetological society about a year ago. They are a male and female both around 7 years old. One is a rescued unwanted snake and the other was a pet. They were put together about 3 years ago and have cohabited ever since. They are of similar size. I personally would not dream of seperating them, they spend their whole time curled up together. They do produce successful young, but I don't see that as a problem. Also I certainly wouldn't add another one.
This bit people may not like. At my Herp Society there is at least one person who houses several corn snakes together. The reason for this is they are all unwanted pets that people don't want, and have been rescued by the society. They do produce eggs ,obviously and he destroys them. Not very nice and probably not very popular on this site. BUT, with so many people breeding corns for certain colours and varieties there are a lot of left over snakes that people don't really want. If someone produces 50 hatchlings in a year and 10 are the colour they want, where do the other 40 go ? A lot end up in a rescue center.
Sorry for going on.