To find 3% of 0.07: $$ \begin{aligned} 0.07 \times 3\% &= 0.07 \times \frac{3}{100} \\ &= \frac{7}{100} \times \frac{3}{100} \\ &= \frac{21}{10000} = 0.0021 \end{aligned} $$ Since $0.0021 = 0.21\%$, the correct option is 0.21%.