how to restrict cited authors to 3 in the Bibliography

[Wolfgang Engelmann]

I use spbasic (Springer Publ) as the bibstyle
(Koma script book)
and get the citations in the text right (Author1, X et al)
However, in the references all Coauthors are cited. How can I resrict it 
to 3?
I tried to find an answer in the net, adding
\usepackage{natbib} to the preamble did not help.
Anybody who could help with a pointer?
Wolfgang