The BeoVision 3 was a 'budget' (and i use that term loosely!) version of the BeoVision Avant. Same tube and chassis, but without the big speakers/ inbuilt DVD/VCR/ rotating stand.
The Avant had the rear connections exposed, and i'd imagine the designers of the BV3 at the time didn't give rear covers much thought. It was only the BV6/BV7 generation of LCD's that gained the rear cover - even the BV5 is 'bare'
The dimensions of the BV3 are on the link here - as are many more on the main site.
The BV3 is a lovely set, and now available to pick up for peanuts compared to new price. Good luck in your search - and post some photos when you get one! 
Lee
BeoWorld - Everything Bang & Olufsen