To find the length, we can use the formula for the volume of a rectangular prism: $$\text{Volume} = \text{length} \times \text{width} \times \text{height}.$$ Substituting in the given values, we have $$90 = \text{length} \times 3 \times 5.$$ Dividing both sides of the equation by $15$ gives $$6 = \text{length} \times 1,$$ and so the length is $\boxed{6}$.
