I don't usually answer questions that seem to be from a class or something...
However, I can tell you the following:
- Constrain: Means that we can't change anything. For example, if a project is time-constrained, then the schedule is fixed and cannot be changed.
- Enhance: Means that we will try to enhance. For example, if a project is cost-enhance, then we will try to enhance the costs by using cost optimization techniques.
- Accept: Means that we accept changes. For example, if a project is scope-accept, it means that we can accept changes (such as reduction) to the scope.
So if, like in your last scenario, we have time-constrained, scope-accept, and cost-enhance, it means that we are accepting scope changes while trying to enhance the costs while at the same time conforming to our fixed schedule.
In any case, the term "Enhance" is not a formal project management term, and it might be understood and explained differently by other project managers.