The roots of polynomial $ p(x) $ are:
$$ \begin{aligned}x_1 &= -0.8241\\[1 em]x_2 &= 0.2454+0.5867i\\[1 em]x_3 &= 0.2454-0.5867i \end{aligned} $$Polynomial $ 3x^3+x^2+1 $ has no rational roots that can be found using Rational Root Test, so the roots were found using qubic formulas.