Answer
It will decrease
Work Step by Step
Mole fraction is (moles of component i / total moles). In the case of mole fraction of B, it would be the (moles of B/ total moles in container). Adding a third gas will increase the total moles in the container, but not the moles of B. This will lead to the denominator getting larger, but not the numerator, which makes the mole fraction smaller.