Quick Answer
A 3rd quartile CASPer result means you performed above the lower half of test takers in your comparison group. It is a positive relative result, but it is not the same as a perfect score, a school-specific cutoff, or an admissions guarantee.
The most practical way to interpret it is this: your CASPer result is unlikely to be the weakest part of your application, but the rest of your file still matters. Admissions committees can still weigh grades, test scores, clinical exposure, service, essays, letters, interviews, mission fit, and school-specific priorities.
If you want the broader score context, read What Is a Good CASPer Score? after this article.