You average the bonuses of all three realms to use against Arcane magic.
For example, the Sample Character in RMX has a Bonus vs Essence of +35, vs Channeling of +0, and vs Mentalism of +35.
Versus Arcane, he would have a RR bonus of +23 (35 + 0 + 35 = 70 / 3 = 23.33333 rounded off to 23).