TRIPLE-C (Time Cost and Manpower Constraints) MODELS:

Until now the discussion of work scheduling has supposed implicitly that resources needed to do the work would always be available. Only scheduling limitation was that predecessor activities must be completed first. A different, additional restriction shall now be considered: constrained resources. A resource refers to working capital or a particular kind of labour, equipment, or material needed to perform activities in the project.

Although several resources are available in sufficient quantity so as not to pose scheduling problems (such as air, unless the project is being conducted under water or in outer space where air is limited), all resources are finite and many are scarce. In several cases, limited availability of skilled workers, machinery, equipment, and working capital dictate that activities must be scheduled at times other than the early, or even late, start date. It is especially true when multiple activities requiring the same resources are scheduled for the same time. When resources are not adequate to satisfy the requirements of them all, some shall have to be rescheduled.

The similar problem occurs in multi-project organizations such as the matrix which depends upon resources from a common pool. To schedule activities for anyone project, managers must take into account resource requirements of other concurrent projects. The result is that project schedules are largely determined by the time when resources will be freed from other higher priority projects.