You can find out either through the MSAR or school websites. Some schools mean 3 years from matriculation and some mean 3 from application. The MSAR is a fantastic resource for many reasons, this being one.
I'm not sure which AAMC policy list you're referring to, but I have a hunch that it's the pdf outlining school policies regarding the new vs. old MCAT, not specific dates. The old MCAT version was proctored last in Jan 2015, so some students may have a score within 3 years of matriculation (year to year not date to date) but still have an old version score. Many schools have elected to only accept new MCAT scores for their 2018 matriculating class - thus the AAMC aggregated these policies.