Well, I think you are correct...
In the Beovision 3 Reference Manual it clearly states
INPUT 1
Digital input socket (S/P DIF) for connection of e.g. a DVD player. INPUT 1 is for digital input from equipment connected to the AV socket on BeoVision 3.
INPUT 2
Digital input sockets (S/P DIF) for connection of e.g. a DVD player. INPUT 2 is for digital input from equipment connected to the V.TAPE socket on BeoVision 3.
If I also review the published connections spec it is:-
V.TAPE - AV - Decoder 3 x 21-pin sockets
V.TAPE : CVBS in/out, RGB in (automatic 16:9 sense (pin-8), B&O AVL)
AV : CVBS in/out, RGB in, S-VHS in/out (automatic 16:9 sense (pin-8), automatic S-VHS configuration, B&O AVL)
DECODER : CVBS in/out (automatic 16:9 sense (pin-8))
BeoLink 1 x Master Link
Camcorder/Auxiliary 3 x Phono sockets (video in/audio L-R in) *) , auto detection
S-Video (S-VHS) 1 x Y/C playback 4-pin socket, auto detection, aspect ratio detection
Headphone socket 1 x Mini jack
Dolby® Digital (optional)
- External BeoLab loudspeakers
- Digital audio input 5 x Power Link (2 x front, 2 x rear, 1 x subwoofer)
2 x Coax phono, Input-1 for AV-scart, Input-2 for V.TAPE-scart
Satellite modul (optional) 2 x F-connector input (2 x 75 ohm)
STB-Controller output (optional) 1 x Mini jack (stereo for 2 x IR-blaster with IR-Y-adaptor)
However, when I review the Service Manual it clearly shows the digital connections as:-
2 x Coax phono, Input-1 for AV-scart, Input-2 for DECODER-scart
So, your problem is that you need RGB on both the AV and decoder inputs because that is where the digital audio inputs are associated, but the DECODER input only has CVBS and NOT RGB
I therefore think your options are limited perhaps to a SCART expander connected to the AV input.