Or in other words E= E(H3AsO4-->H3AsO3) (reduction potential)
Shouldn't be E = -E(H3AsO4--> H3AsO3),because H3AsO3/H3AsO4 is the anode!!?
yes it is oxidation and if reduction potential is given then you need to take reverse of it.
If the formula is Ecell = Ecathode + E anode
however is you are using this formula
Ecell = Ecathode - E anode
Then you need to take reduction potential as such as negative of the formula is making it oxidation potential.