To calculate the focal length of lens we use the formula:

\(\rm P \) = $\frac{1}{f}$

where \(\rm P\) = Power of lens

and,   \(\rm f\) = focal length of lens

For P = -3D :

\(\rm P \) = $\frac{1}{f}$

\(\rm or,\) \(\rm f\) = $\frac{1}{P}$

\(\rm or,\) \(\rm f\) = $\frac{1}{-3}$

\(\rm or,\) \(\rm f\) = \(\rm -0.3333m \)

Hence, for lens with  power of lens -3D focal length is -0.3333m or -33.33cm approximately. This indicates it is a concave lens, typically used for correcting nearsightedness.