This concept has been replaced with 'competing project constraints', 'project constraints' or simply 'constraints'.
The triple constraints was a simplified approach. However, starting with PMBOK Guide Fifth Edition PMI realized that it's not just 3 constraints, but there can in fact be many constraints: Scope, schedule, cost, quality, resources and risk (and more!) all compete for our attention, our money, our time, our everything. So forget the "triple" constraints and welcome "project constraints" that all pull for your attention.