I don't know anything about it in terms of probability, but I have heard different things from different people. I think it depends on the program. A lady from one of the UTs told me that, in her program, if you're invited it means you're in. That's not the case at all schools, though. I've heard that some PIs will rank the applicants they invite, and if one is their first choice and continues to be their first choice after the interview weekend, the other two don't stand much of a chance (unless, of course, the first choice chooses to go elsewhere).
Because of this, I don't think that thinking of it in terms of probability/chances is wise unless you know for certain that the PI likes each interviewee equally and the interview is to try and become "first choice," as opposed to a situation where a first choice is already pretty much chosen and the interview is just the PI's way to confirm his/her choice.