Hello Lars,
Welcome to the forum!
Beolab 3's have power amps built in, whereas Beolab 5's have both pre AND power amps built in. Hence, you can use Beolab 5 on Line Out (fixed volume) outputs from your receiver and control them via their own pre-amp using a Beo4 remote, whereas the Beolab 3's need to be connected to a variable (volume controlled) pre-out output on the receiver.
Normally, the best option is to use both sets of speakers on the pre-out sockets and pre-set the Beolab 5 volume to a suitable level (normally 50-75%) so that they are all controlled from the receiver remote and the volume levels match between the speakers.
Beolab 5's use standard RCA cables when in Line mode (although they do need a special Sync cable between the two speakers to keep the volume levels balanced). However, Beolab 3's need a special RCA to DIN cable with custom wiring inside the DIN plug so that they work correctly. If you got your cable from a generic (ie. non-B&O specialist) cabling store, then you can be fairly sure that your Beolab 3 cables will be incorrect.
If you need the correct cables, please click on my link at the foot of this page and I will be happy to provide these (with a 60 day money-back promise, so you won't be left with any more unwanted cables if you change your mind!)
Kind regards,
Steve.
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