The Triangle Concept depicted the interdependency of these constraints. i.e scope changes lead to changes in Time, Cost and Quality issues, similar changes in any one constraints affects the others significantly..
Modern Day Project Management have more Project constraints like
- SAFETY - Zero Accident Free Manhours,
- Stakeholder Requirements / Satisfaction,
- Social Responsibilities,
- Legal and Regulatory Requirements,
- Environmental Impacts,
- Skilled Resource Constraints
- Technological Advancements
Hence a present day Project Constraints could only be depicted in the form of a 'n' sided Pyramid based on 'n' Project Constraints under consideration for a given Project with "QUALITY" being the BASE of the PYRAMID.
Well, still if your project constraints falls only in a Project Management Triangle Format then, you have a well defined Project with all other constraints already taken care off or inbuilt.