Soll solution is an ion conductor. Electron transfer and therefor redox reactions are restricted to the electrodes. Thus Cr III oxidation in the soil compartment is unlikely.

cr6 + cr3 = total cr. now, the analytical methods can only test either cr6 or total. but you can calculate the cr3 concentration based on basic chemistry if you have both total and cr6.

I agree. But in a close to the electrodes if it could get the Cr (VI) (on simple diffusion)!