A 3.5 GPA is below UC Santa Barbara's admitted middle-50% GPA range of 3.9–4.18.
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 33.0%-acceptance school like UC Santa Barbara. 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.