Fulfil - that is, Make full. I have joy in you, complete it by that which is still wanting, namely, unity (Phi 1:9).
likeminded - literally, “that ye be of the same mind”; more general than the following “of one mind.”
having the same love - equally disposed to love and be loved.
being of one accord - literally, “with united souls.” This pairs with the following clause, thus, “With united souls, being of one mind”; as the former two also pair together, “That ye be likeminded, having the same love.”