\(\dfrac{2x+1}{3}-\dfrac{x-1}{2}< 3\)
\(\Leftrightarrow\dfrac{2\left(2x+1\right)-3\left(x-1\right)}{6}< \dfrac{18}{6}\)
\(\Leftrightarrow2\left(2x+1\right)-3\left(x-1\right)< 18\)
\(\Leftrightarrow4x+2-3x+3< 18\)
\(\Leftrightarrow x< 13\)
Vậy \(S=\left\{x|x< 13\right\}\)