A project triangle. Similar to the sign occasionally seen in stores asking you to pick two of cheap/fast/well done.
Other examples from the link:
- College: Work, Sleep, Play
- Men: Handsome, High-Earner, Faithful
- Women: Single, Sane, Sexy, Smart – Pick any three
- Operating system: Fast, Efficient, Stable
- Bicycle Parts: Strong, Light, Cheap