To solve for \( x \) in the ratio \( \frac{x}{8} = \frac{y}{16} \) when \( y = 3 \), we can substitute the value of \( y \) into the equation.
First, plug in \( y = 3 \):
\[ \frac{x}{8} = \frac{3}{16} \]
Next, we will cross-multiply to solve for \( x \):
\[ x \cdot 16 = 3 \cdot 8 \]
Calculating the right side gives:
\[ 16x = 24 \]
Now, divide both sides by 16 to isolate \( x \):
\[ x = \frac{24}{16} = \frac{3}{2} = 1.5 \]
Therefore, the value of \( x \) is 1.5 units.