A 3.0 GPA is below Saint Mary's College of California's admitted middle-50% GPA range of 3.25–3.7.
On GPA alone this is a reach — to be competitive you'd want strong test scores, a demanding course load, and a standout application to offset it.
A GPA is the single biggest academic factor, but it's necessary, not sufficient — especially at a 86.0%-acceptance school like Saint Mary's College of California. Your test scores, course rigor, extracurriculars, essays, and any standout achievements all move the number. The estimate above assumes typical scores and rigor; your real odds depend on your full profile.
Estimates are CDS-based and for a typical applicant. Run your full profile for your real, calibrated odds.