Given that the figure is a rectangular prism, we have to use the following formula
[tex]S=2lh+2hw+2lw[/tex]Where, l = 12, w = 8, and h = 3.
[tex]\begin{gathered} S=2\cdot12\cdot3+2\cdot3\cdot8+2\cdot12\cdot8 \\ S=72+48+192 \\ S=312(cm)^2 \end{gathered}[/tex]