The fraction $31\dfrac{3}{8}$ converted to decimal is

1. 24.125
2. 24.625
3. 31.375
4. 31.675

$31\dfrac{3}{8} = 31 + \dfrac{3}{8}$

Dividing 3 by 8, we get:

$\begin{array}{r} 0.375 \ 8 \overline{\smash{)} 3.000 } \ \underline{24} \phantom{.00} \ 60 \phantom{.0} \ \underline{56} \phantom{.0} \ 40 \phantom{} \ \underline{40} \phantom{} \ 0 \phantom{} \end{array}$

31 + 0.375 = 31.375

Hence, option 3 is the correct option.