If you have to do 70% of your projects and you have 3, you can approach this problem by showing that each project is 33.3% (I did 100/3 to determine what percentage each project is worth).
If we do one project, 33.3% is completed
If we do two projects, 66.6% is completed
If we do three projects, 100% is completed
Since we have to do at least 70%, we must do all three projects.