How do you say tertiary in English?

"Tertiary" is already the English word for this concept. It's used in various contexts, like:

* Tertiary education: referring to higher education levels like universities and colleges.

* Tertiary sector: the part of the economy that focuses on services, like retail, finance, and healthcare.

* Tertiary structure: the three-dimensional shape of a protein.

It's a perfectly fine word in English, so you don't need to change it.