I am configuring a beosystem 1 with 2 speakers in front and 2 in rear, no center

When I start the tv and select menu > setup > sound 

Speaker mode > video is either speaker 3 or speaker 5

Speaker mode > audio is either speake 2 or speaker 4

Active sound system is set to dolby 3 stereo (and cannot be changed)

So every time I start the tv I have dolby 3 which since I dont have a center speaker connected sounds well.. not good

Can this default thing be changed?

Active sound system shows the sound format beeing delivered from the source. Obviously your source is playing dolby 3. To change this information you have to reconfigure your source, not the BS1.

I can change it via beo4 display in "speaker" and press i.e "2" but the default on cable-tv is always stereo3 and that source is "a cable" so I cant change that in any way.

It says in the manual that only speaker 3 and 5 is available on video so I guess beo is assuming everybody purchase a center speaker :)

I will probably do that also anyway so.. but it seems a bit limiting with this function

Sorry I missunderstood.

I think there is no way to change the selection and your assumption is right. But I definitly would suggest to add a center speaker.