Let for \( x \in \mathbb{R} \), \( S_0(x) = x \), \( S_k(x) = C_k x + k \displaystyle\int_0^x S_{k-1}(t)dt \), where \( C_0 = 1, \; C_k = 1 - \displaystyle\int_0^1 S_{k-1}(x)dx, \; k = 1, 2, 3, \ldots \) Then \( S_2(3) + 6C_3 \) is equal to _____.
