To solve the equation \( \frac{x}{8} = \frac{y}{16} \) for \( x \) when \( y = 3 \):
- Substitute \( y = 3 \) into the equation:
\[ \frac{x}{8} = \frac{3}{16} \]
- Cross-multiply to solve for \( x \):
\[ 16x = 3 \cdot 8 \]
- Calculate \( 3 \cdot 8 \):
\[ 16x = 24 \]
- Divide both sides by 16 to solve for \( x \):
\[ x = \frac{24}{16} = \frac{3}{2} = 1.5 \]
Therefore, the value of \( x \) is 1.5 units.