The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= 3.4101\\[1 em]x_2 &= -0.2051+2.0872i\\[1 em]x_3 &= -0.2051-2.0872i \end{aligned} $$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.