Background: A three-pronged approach to acne treatment—combining an antibiotic, antibacterial, and retinoid—could provide greater efficacy and tolerability than single or dyad treatments, while potentially improving patient compliance and reducing antibiotic resistance. Objectives: We aimed to evaluate the efficacy and safety of triple-combination, fixed-dose topical clindamycin phosphate 1.2%/benzoyl peroxide (BPO) 3.1%/adapalene 0.15% (IDP-126) gel for the treatment of acne. Methods: In a phase II, double-blind, multicenter, randomized, 12-week study, eligible participants aged ≥ 9 years with moderate-to-severe acne were equally randomized to once-daily IDP-126, vehicle, or one of three component dyad gels: BPO/adapalene; clindamycin phosphate/BPO; or clindamycin phosphate/adapalene. Coprimary endpoints were treatment success at week 12 (participants achieving a ≥ 2-grade reduction from baseline in Evaluator’s Global Severity Score and clear/almost clear skin) and least-squares mean absolute changes from baseline in inflammatory and noninflammatory lesion counts to week 12. Treatment-emergent adverse events and cutaneous safety/tolerability were also assessed. Results: A total of 741 participants were enrolled. At week 12, 52.5% of participants achieved treatment success with IDP-126 vs vehicle (8.1%) and dyads (range 27.8–30.5%; P ≤ 0.001, all). IDP-126 also provided significantly greater absolute reductions in inflammatory (29.9) and noninflammatory (35.5) lesions compared with vehicle or dyads (range inflammatory, 19.6–26.8; noninflammatory, 21.8–30.0; P < 0.05, all), corresponding to > 70% reductions with IDP-126. IDP-126 was well tolerated, with most treatment-emergent adverse events of mild-to-moderate severity. Conclusions: Once-daily treatment with the novel fixed-dose triple-combination clindamycin phosphate 1.2%/BPO 3.1%/adapalene 0.15% gel demonstrated superior efficacy to vehicle and all three dyad component gels, and was well tolerated over 12 weeks in pediatric, adolescent, and adult participants with moderate-to-severe acne. Clinical Trial Registration: ClinicalTrials.gov identifier NCT03170388 (registered 31 May, 2017).