Why cyclic voltammetry requires three electrodes?
Normally, you want the applied potential difference between working and reference electrode not to change. When you use two electrode setup, as chemical reaction occurs, there is some current (faradaic) passing through reference electrode which when combined with reference electrode resistance changes the potential at which reference electrode stays, thus changing the potential difference. Therefore, third electrode is introduced to the electrochemical cell. In Cyclic voltametry (CV), the potential is not changing based on reaction occuring (you determine the upper and lower limits for scan). The position of electrodes for CV is important when the solvent is resistive and it is rule of thumb to put reference close to working and not to put counter between them.