The Beogram 3500 should have a captive 7 pin DIN plug. It rather depends on which Beomaster 3000 you have. I imagine you have one of the slide rule receivers - very nice too! This has a 5 pin DIN socket for Tape 2 which is the input I would use. You need to unscrew the two extra pins in the CD3500 plug. Keep them safe!
Alternatively, you might have a later touch sensitive 3000. This will also work via the Tape or Tape 2 sockets.
Usual suggestion for speakers will normally start with S45-2s which will work very well. 3800s are rather nice but a UK only model and not that common.