If all three are from the same location then the risks are much, much lower. I was under the impression that the previous snakes were long term captives that have been dewormed, ect.
Reptile diseases are much less studied than human diseases, but unless a significant amount of time has surpassed between captives the odds are again in your favor. An example of a more famous disease that is spreading is the deadly chytrid pathogen in the frog world.
It is considered good practice to always quarantine new animals prior to mixing with a collection. Of course any risk will vary upon the situation and it is up to the individual to decide if quarantine is something they want to do. Because it's not hard to do, and I'm anal retentive when it comes to herp health, it is somthing that I always do for my animals.
Ian