Separation and qualitative identification of cations
Well there is a pretty simple way, that you can do in any laboratory by standard reagents too.
React the unknown mixture with $\ce{NH4OH + NH4Cl}$
(ammonium chloride is put as a standard to prevent the precipitation of Zn, Mn, Cu and Ni hydroxides, however, you don't have those in your sample, so you can proceed with only ammonium hydroxide too)
now you will observe precipitate as

Green- confirms $\ce{Cr^{+3}}$

Reddish-brown- confirms $\ce{Fe^{+3}}$

White- confirms $\ce{Al^{+3}}$

(These are colours of the corresponding hydroxides)

Now if you have multiple salts, you carry out the following conformatory tests

For $\ce{Cr^{+3}}$, add a few dorps of strong oxidising agent in an acidified medium ($\ce{KMnO4}$/$\ce{H+}$ would do), this oxidises $\ce{Cr^{+3}}$ to $\ce{Cr2O7^{-2}}$ which gives the super famous chromyl chloride test.

For $\ce{Fe^{+3}}$, add a few drops of $\ce{CH3COONa}$ and you will initially observe a dark-red solution of $\ce{Fe3(OH)2(CH3COO)6}$, which on heating precipitates as blood red $\ce{Fe(OH)2CH3COO}$ 

For $\ce{Fe^{+2}}$ simply add KI, very distinct black $\ce{FeI2}$ precipitates out 

For $\ce{Al^{+3}}$ add ammonium phosphate, you will see a cloudy white precipitate 

You can read more here, Vogel is pretty much the most solid and trustworthy book for practical inorganic chemistry
You are welcome!