You're trying to plan too much. You have one on the hook and you're going to ruin it because you are more interested in another girl. If you mess things up with girl B, then that will ruin things for you with girl A. Don't believe that because she says she's fine with you going after A that she's really okay with it (Her response seemed to try to discourage you and not to show any signs of jealousy). Keep doing what you're doing with B and lay the groundwork to getting B to invite A to join the two of you.
Thanks bro, i was thinking i could build some rapport with Girl A in order to gain her trust and then get B to do the job, but what could i do to get B to invie A to join us?