Since \( y \) varies directly with \( x \), we can express this relationship with the equation:
\[ y = kx \]
where \( k \) is the constant of variation.
From the information given, when \( x = 3 \), \( y = 15 \). We can use this information to find \( k \):
\[ 15 = k \cdot 3 \]
To find \( k \), we divide both sides by 3:
\[ k = \frac{15}{3} = 5 \]
Now that we have the value of \( k \), we can write the direct variation equation as:
\[ y = 5x \]
Next, we want to find \( y \) when \( x = 21 \):
\[ y = 5 \cdot 21 \]
Calculating this gives:
\[ y = 105 \]
Thus, when \( x = 21 \), \( y \) is \( \boxed{105} \).