To find out what percent $3 is of $10, you can use the formula:
\[ \text{Percentage} = \left( \frac{\text{Part}}{\text{Whole}} \right) \times 100 \]
In this case, the "Part" is $3 and the "Whole" is $10.
So, the calculation will be:
\[ \text{Percentage} = \left( \frac{3}{10} \right) \times 100 \]
Calculating this:
\[ \frac{3}{10} = 0.3 \]
Now multiply by 100:
\[ 0.3 \times 100 = 30 \]
Therefore, $3 is 30% of $10.
The final answer is \( \boxed{30%} \).