If the goal of the question is to prove that $f(a,b) = \int_0^\infty e^{-ax^3-bx^2}\mathrm dx,$ satisfies $3ab\frac{\partial^2 f}{\partial b^2}-3a\frac{\partial f}{\partial b}-2b^2\frac{\partial f}{\partial a} = 1,$ then this can be done by looking at all terms at the same time, not separately.
I will assume that the differentiating under the integral sign is not the problematic part.
By plugging in, we get the following: $3ab\frac{\partial^2f}{\partial b^2}-3a\frac{\partial f}{\partial b}-2b^2\frac{\partial f}{\partial a} = \int_0^\infty \left(3abx^4+3ax^2+2b^2 x^3\right)e^{-ax^3-bx^2}\mathrm dx.$
Now, observe that $\int_0^\infty 3ax^2e^{-ax^3 - bx^2}\mathrm dx = \int_0^\infty ax^3\left(3ax^2 + 2bx\right)e^{-ax^3-bx^2}\mathrm dx ,$ by integration by parts.
This means that, after factoring, we obtain $\int_0^\infty \left(3abx^4+3ax^2+2b^2 x^3\right)e^{-ax^3-bx^2}\mathrm dx = \int_0^\infty \left(3ax^2+2bx\right) \left(ax^3+bx^2\right) e^{-ax^3-bx^2}\mathrm dx.$ Using the substitution $u = ax^3 + bx^2$, this reduces to $\int_0^\infty ue^{-u}\mathrm du = 1,$ whence we get the wanted identity.