The 3 constraints is now a nearly defunct term and it is no longer mentioned in the PMBOK (version 4). Anyway, the 3 constraints are: Time, Scope, and Cost.
Time means the duration of the project (or the schedule)
Scope is the collection of all the activities and the deliverables for the project.
Cost means the total budget of the project (which includes the costs of labor, and other project related costs)
The 3 constraints always interact with each other, for example, if we reduce the cost of the project, then most probably we will reduce the scope. If we increase the time, then we will increase the cost.
Before the 3 constraints were seen as each representing the angle of a triangle, if one angle is changed then at least 1 of the other 2 is changed as well. However, this theory proved to be incorrect (no idea who created the theory of the "project triangle" concept). For example, it is not necessary for the costs to decrease if we reduce the scope (in fact, in most cases, the costs will increase even if the scope is reduced, or at least stay the same).
Project Managers usually give a weight to each of one of these constraints in their projects. The weight is anywhere from 0 to 1, the total weight of all these constraints should be 1. For example, the Project Manager might decide to give a weight of 0.2 to the Time, 0.3 to the Cost, and 0.5 to the Scope. This means that the scope is the most important constraint in the project, while time is the most flexible (in other words, we have leeway when it comes to the deadline, but we have to deliver exactly according to the scope, we cannot play with quality).