Hi chrisped,
You need a stereo 3.5mm mini-jack to connect to the PC and two 7 pin DIN plugs for the Beolab 3's. Don't use a cable with 5 pin DIN plugs fitted, as this won't have the voltage reference pin connected, so it may cause humming noises through the speakers.
Some dealers are selling a combination of three cable adapters to do this job, which can be rather unsightly in some applications. A little bit of searching (eBay, internet, etc) will get you a high quality cable without any need for adapters. If you prefer to buy from a franchised dealer, you may want to ask them if they sell cables other than the stock B&O offerings, as the choice of companies supporting this market is much wider than it was a couple of years ago.
Best regards,
Steve.
Sounds Heavenly Cables are proud to be a sponsor of the BeoWorld Forum!