Sometimes only time, scope and cost listed. Sometimes quality is added too – which I think is a valuable addition. Any fool can produce something to cost, time and scope, but it might not work well. Therefore suitable quality is an essential component for a successful project.
I think it is separate from scope, though probably emerged from scope. You might say that an outcome of an inventory system is to place orders automatically when stock falls to a certain level. Quality adds conditions like reliability, timeliness, ease of use.