The linear density of a fiber is proportional to the square of its diameter. Therefore, if the ratio of the linear densities is 3, the ratio of the diameters \( D_1 \) and \( D_2 \) is given by:
\[
\frac{D_1}{D_2} = \sqrt{3} \approx 1.732.
\]
Thus, the ratio of the diameters is \( 1.73 \).