yeah thanks, i already had configured MRL/MRG with correct associations.
Well, indeed i solved the problem and its kind of embarrassing:
The thrid participant was not using his jabber as a cti devices (1 and 2 were), he was using it as a softphone.
Of course he had no CSF-Profile for it.
After also the third participant configured his jabber in deskphone mode it worked out fine.
Thanks for your effort!