The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= -1.5198\\[1 em]x_2 &= 2.2599+2.1823i\\[1 em]x_3 &= 2.2599-2.1823i \end{aligned} $$Polynomial $ x^3-3x^2+3x+15 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.