  Let $I \subset R = \mathbb{F}[x_1,x_2]$ be a height two ideal minimally
generated by three homogeneous polynomials of the same degree $d$, where
$\mathbb{F}$ is a field of characteristic zero. We use the theory of
$D$-modules to deduce information about the defining equations of the Rees
algebra of $I$. Let $\mathcal{K}$ be the kernel of the canonical map $\alpha:
\text{Sym}(I) \rightarrow \text{Rees}(I)$ from the symmetric algebra of $I$
onto the Rees algebra of $I$. We prove that $\mathcal{K}$ can be described as
the solution set of a system of differential equations, that the whole bigraded
structure of $\mathcal{K}$ is characterized by the integral roots of certain
$b$-functions, and that certain de Rham cohomology groups can give partial
information about $\mathcal{K}$.
