I never had the pleasure to try (the one time I was at LHR T3 last year I missed a connection and had other things to manage), but guess there must be a lounge for KL and AF at T3 since they used it for a while with Virgin not yet in Skyteam. Yes, know the T4 lounge from the time before that. But my suggestion was which lounges KL and AF used after the change to T3.