In mathematical speak, "three over eight" is a valid term.
So are:
13/18 -------> Thirteen over eighteen;
24/26 -------> Twenty-four over twenty-six.
In mathematical speak and in everyday usage, these are also valid terms:
3/8 --------> Three eighths
13/18 -------> Thirteen eighteenths
24/26 --------> Twenty-four twenty-sixths
In American English:
3 1/4 --------> Three and one quarter
1 1/2 --------> One and one half
Note the differences in British English:
3 1/4 --------> Three and a quarter
1 1/2 --------> One and a half
Whichever system you choose to use, just be consistent.