\[a_{1} = 3;\ \ a_{2} = 6;\ \ldots;\ \ a_{n} = 78.\]
\[a_{n} = 3 + (n - 1) \cdot 3\]
\[78 = 3 + 3n - 3\]
\[3n = 78\]
\[n = 26.\]
\[S_{26} = \frac{a_{1} + a_{26}}{2} \cdot 26 = 13 \cdot (3 + 78) =\]
\[= 13 \cdot 81 = 1053.\]
\[Ответ:1053.\]