  Unstable modes growing when two plasma shells cross over a background plasma
at arbitrary angle $\theta$, are investigated using a non-relativistic three
cold fluids model. Parallel flows with $\theta=0$ are slightly more unstable
than anti-parallel ones with $\theta=\pi$. The case $\theta=\pi/2$ is as
unstable as the $\theta=0$ one, but the fastest growing modes are oblique.
While the most unstable wave vector varies with orientation, its growth rate
slightly evolves and there is no such thing as a stable configuration. A number
of exact results can be derived, especially for the $\theta=\pi/2$ case.
