The volume of a rectangular prism can be calculated with the formula

To find the width of the rectangular prism, we can rearrange the volume formula \( V = lwh \) to solve for width \( w \).

Given:
- \( V = 90 \, \text{ft}^3 \)
- \( l = 3 \, \text{ft} \)
- \( h = 12 \, \text{ft} \)

We can plug in the values into the formula and solve for \( w \):

\[
90 = 3 \times w \times 12
\]

Calculating the product of length and height:

\[
90 = 36w
\]

Now, we can solve for \( w \) by dividing both sides by 36:

\[
w = \frac{90}{36}
\]

Calculating the fraction:

\[
w = 2.5
\]

Thus, the width of the rectangular prism is \( \boxed{2.5} \) ft.