When we have two forms of As in an environment, how and in which potential does the reduction of As+3 to As0 occur, and the interference of As+5 form is zero?
It looks like the overpotential between the two processes above is not too bad however, ability to selectively reduce As 3+ may depend on the capabilities of your working electrode. I would suggest you prepare separate solutions of the metal in the two oxidation states and run a CV or another technique to see their reduction potentials at your WE. Then you can prepare a solution combining both forms and run the experiment again to check selectivity. You should be able to find an optimum potential for the reduction studies from these experiments.