Oxidation State in Tetraaquadichlorochromium(III) chloride
I think the easiest way to see this is to count the formal charges on the ligands.

The four waters do not have net charge.  The remaining ligands--the three Cl's--are likely $\ce{Cl-}$, and so their total charge contribution is $-3$.

The only way the complex can be neutral is if the chromium has a $+3$ charge:  hence Chromium (III).