The three point editing is an editing method which is done by setting three edit points. The first and second point are the in and out points of the clip. In a timeline were the clip fits into they are the in and out points. The third point allows both in and out for the clip. Three point editing enables editors to use both source clip and sequence in and out points to specify the duration of a source clip. in Final Cut pro the fourth point is automatically inferred.
This picture shows the in and out points with source clip. in order to insert the in point a computer keyboard can be used to move the playhead one frame at a time to the place were the in point will be placed.
This is very simple way to edit and will speed up your workflow. The best way to get familiar with the three-point edit is to practice it and you more than likely end up using it for almost all of your sequences.