We have 3 iPhone 4s - me, hubby and daughter. Since we uploaded iOS6, many of the texts from hubby and daughter have been mixed up -- she gets texts from his friend; her texts to me appear to come from him; his texts to me go to her, etc. It's very annoying and it means that hubby and I can't discuss the kid in private! I've talked to one other person having this problem.
Hubby thinks it is because he and daughter share an iTunes account and email address. He tried to change his iTunes account address this past weekend and it wouldn't let him.
Check the settings on all 3 devices and make sure you all don't have multiple Send & Receive addresses that match. Go to Settings -->Messages-->Send & Receive. If, for example, the iTunes/Apple ID email address is one of the "You can be reached by..." selected choices on all three devices then that may be the culprit. Hope that makes sense.