Number of moles = Mass / Molar mass​

Molar mass of NaOH=$40$ g/mol

By Using  the definition of molarity to find the volume of the solution:

Molarity= $\frac{\text{Number of moles of solute}}{\text{Volume of solution (in liters)}}$

$$\begin{gathered} \mathrm{M}=\frac{{\mathrm{n}}_{\mathrm{NaOH}}}{{\mathrm{V}}_{\mathrm{sol}}\left(\text{ in }\mathrm{L}\right)} \\ \Rightarrow 3=\frac{(84/40)}{\mathrm{V}} \\ \Rightarrow \mathrm{V}=0.7 \mathrm{L}=7\times {10}^{-1} \mathrm{L} \end{gathered}$$