the 'threesome' is becoming established
And there lies the problem. If you back off at times, then the new 'twosome' will start to become established. But I don't see what else you can do. If you invite L out for coffee, or to your house for lunch, she will very likely - and innocently expect the interloper to have been invited as well. You can hardly say to her, "oh, by the way, don't say anything to X", so she may well mention it to her. If this X is a seasoned 'Wendy', then she will act the injured party and seek L's sympathy and support. This could even lead to L being encouraged to see you in a new light, someone who is, for no reason, being hurtful to X.
But for all that, you should withdraw at times and, more importantly, find other friends. As Wendysue says, that will help you if L is completely taken over by this woman, though I too hope it doesn't come to that. And if L is a true and longstanding friend, it is unlikely to come to that.
Good luck 