In high intensity circular accelerators such as Rapid Cycling Synchrotrons (RCS) and Fixed Field Alternating-gradient (FFA) synchrotron, maximum beam intensity is limited by beam loss. Suppression of beam scatterings occurred in beam extraction system, especially in septum magnets, is one of the key issues to achieve low beam loss rate. Application of massless septum may be a promising way to overcome the beam loss problem. In this study, three dimensional design of the massless septum magnets for RCS and FFA is investigated. Utilizing 3D simulations, effects of nonlinear field distribution on extracted beam are analyzed and requirement of the massless septum magnets will be discussed.