This preprint introduces the Musical Aptitude Triad, a framework for conceptualising musical ability as three interdependent domains: tonal, performative, and expressive. Each domain is linked to distinct neurocognitive substrates—auditory/working-memory systems, motor and cerebellar timing systems, and affective–communicative circuits—while musical competence in practice emerges from their integration. The model incorporates a developmental dimension (the Developmental Expansion Index) to represent how aptitude expands with training and experience. The Triad is designed to support differentiated pedagogy, research operationalisation, professional role alignment, and clinical/therapeutic applications, with illustrative analogies to commercial music roles (producer, session player, artist/songwriter).