General synonyms:
* Third
* Secondary (in some contexts)
* Subordinate
* Lower-level
* Minor
* Inferior
Specific synonyms:
* Third-order (in terms of classification or hierarchy)
* Intermediate (in terms of position or importance)
* Derived (in terms of origin or development)
Examples:
* Tertiary education: Higher education (e.g., college or university)
* Tertiary industry: Service industry
* Tertiary structure: The three-dimensional shape of a protein
The best synonym for "tertiary" will depend on the specific context in which it is used.