Let me first say that I am not considering housing more than one snake together. I believe them to be most content when solitary.

However, while surfing craigslist I came across a person rehoming a large tank with three corn snakes in them. Two of the corns were the same size and the third was smaller. Is this safe?

Some species can be housed together and there are notable examples of wild snakes sharing dens (rattlesnakes, copperheads, and so on) with others of their species as well as other species of snake. Rattlesnakes also have been known to share dens with tortoises. There are also well documented examples of garter snakes sharing dens as well. If the animals are not cannibalistic or antagonistic in nature than it may be reasonable to house them together. It is not so much an issue as long as the enclosure is large enough to do so and is able to provide multiple basking and hiding spots so less dominant animals are not forced to accept less than ideal conditions. Obviously in species where males combat for breeding rights males should not be housed together (corn snakes will do this). In any environment in which multiple animals are housed together diligence on behalf of the keeper is necessary to assure all the animals are thriving. If antagonistic behavior is noticed or one animal is showing obvious signs of stress it should be relocated into it's own enclosure.

Cannibalism is not unheard of. I would strongly recommend NOT housing them together! Males will fight with each other. They don't bite at each other, but they will "impose their will" by using their bodies to push each other around. This will result in a stressful environment and will lead to snakes that will refuse food and develop illnesses. If males and females are housed together, the male will continue to mate with the females until they exhaust themselves and will expire due to over exertion. If females are housed together, then they will live a little longer, but it still is not recommended. All snakes thrive better when in separate housing.

yes they dont hurt each other some snakes in the wild will actually stay with there siblings in the same den they leave for food but come back to the same den for the night or even winter

yes you xcan maaaaaan

simple mate yes you can, ,no need for the lectures of the main contributer just dont mix breads as this will make matters worse

well its just like humans some people get along better some fight like cats and dogs