Notorious did a bang up job of outlining the rules of shitequette a while back. (Jeebus, looking at the picture still makes me sick) I feel like I have to add a caveat, though to smaller bathrooms.
It's simple arithmetic...one, two, three...(You give us the tapes, we take the tapes to the record company, we get a record contract, we pay you back your money. We're guaranteed to get a record contract. Our stuff's that good. OK, I don't know this industry jargon, YP, MP, all I know is that the magic, the heart and soul that is on those tapes...that's ours, and you don't own that.) Sorry, I got carried away.
If there are 3 shitters (same rule for urinals, incidentally) then this should be easy. If you go in and there is no one in there, then you may choose either shitter #1 or shitter #3. You MAY NOT, barring wide scale grossness in both other stalls, choose #2. What the eff, people? This should be easy. Then if there is one person in there, presumably in #1, then you should go to #3. Simple concept.
The only time you may choose #2 is when both #1 and #3 are taken. The caveat is if someone absolutely destroyed one of the end shitters and didn't clean up after themselves leaving it clogged or otherwise unusable. You may not break this rule even if there is a moderate cleanup effort involved before you can get down to business. You're going to have to determine for yourself the difference between moderate and unreasonable cleanup job. Follow the fucking rules, people.