You are looking for a Powerlink to RCA cable. As the the description implies it consist of a powerlink connection on one end which would go to the back of the BeoLab 3's and the other side would be a male RCA connection, which would be ready for most all preamp/monitor connections. It is not a Y connection so you would need two. As far as preamp's go I would fine one with controls that fit your prefaces. Stick with higher quality lines and you should be just fine.
BV7-40 MKIV, BeoVision Avant 32, BeoVision 8-32, BeoLab Penta MKIII, BeoLab 6000 x 3, BeoSound 9000 MKIII, BeoLab 7-4, BeoSound 4000, BeoPort, BeoVox S45-2, ML/MCL Convertors, A8's & Form 2's.