To convert the ratio \( 12:36 \) into an equivalent ratio of \( x:3 \), we can set up a proportion based on the given ratios:
\[ \frac{12}{36} = \frac{x}{3} \]
First, we simplify the left side of the equation:
\[ \frac{12}{36} = \frac{1}{3} \]
Now, our equation becomes:
\[ \frac{1}{3} = \frac{x}{3} \]
By cross-multiplying, we can solve for \( x \):
\[ 1 \cdot 3 = x \cdot 1 \implies 3 = x \]
Thus, the equivalent ratio is:
\[ \boxed{3:3} \]
Or simply \( 1:1 \) when reduced to the simplest form.