homework and exercises - How to find constant of motion for Hamiltonian system?

I have to find a constant of motion associated to this Hamiltonian but I don't know how to proceed.

$$H=\frac{\mathbf{p_0}^2}{2m}+\frac{\mathbf{p_1}^2}{2m}+\frac{\mathbf{p_2}^2}{2m}-2V(\mathbf{r_1}- \mathbf{r_0})+V(\mathbf{r_2}-\mathbf{r_1})$$

where $$V(\mathbf x)=\frac {e^2}{|\mathbf x|}.$$

I don't know what $\mathbf x$ is.

This Hamiltonian refers to a system of 3 particles $(0,1,2)$ with mass $m$ and charge $e$.

The coordinates are $r^\alpha_i$ and conjugate momenta $p^\beta_j$ with $\alpha, \beta=0,1,2$.

I have written all the information that I have.