Returns difference in RVs (separated by time dt) of star 3.

def dRV_3(self,dt):
        """Returns difference in RVs (separated by time dt) of star 3.
        """
        return -self.orbpop_long.dRV(dt) * (self.orbpop_long.M1/(self.orbpop_long.M1 + self.orbpop_long.M2)) -\
            self.orbpop_short.dRV(dt) * (self.orbpop_short.M1/(self.orbpop_short.M1 + self.orbpop_short.M2))