Answer
The quotient of b and 3.
Work Step by Step
Two things are being divided, so we say the phrase "the quotient of" and then list the two things being divided, in this case b and 3. Note, because we want the resulting fraction to be b/3, not 3/b, we list the b first, saying the quotient of b and 3 instead of the quotient of 3 and b.
