I don't think it is a one in three chance at all. Most schools send secondaries to everyone who they receive primaries from. It is called, "give me more of your damn money and send us some letters". Other schools screen applicants based on gpa, mcat, personal statement, background, etc and only send secondaries to less than half of their original applicant pool. I know that UCSF narrowed its applicant pool from around 4500 to 1400-1500 by looking at the above mentioned factors. So, 1400-1500 received secondaries. Out of those, around 500 are interviewed. So, if you get a secondary you have a 1 in 3 chance of getting an interview. Once interviewed, about 200 of the 500 are given outright acceptances. Most of the UC's work this way as well, but start off with different numbers. Stanford also heavily screens. Their interview pool is smaller than UCSF's, but I think I remember that they sent a similar amount of secondaries.
Bottom line..a secondary does not mean you have a 1 in 3 chance at all, but if they screen it is a really good sign. Your chance of acceptance has increased quite a bit..at this point, you can't ask for much more.