What is the word phrase for 3r-1?

There isn't a specific word phrase for the mathematical expression "3r - 1". It's best described as:

* Three times 'r' minus one.

* The product of three and 'r', minus one.

You could also say:

* 'r' tripled, then reduced by one.

Ultimately, the best word phrase depends on the context and the desired level of formality.