biblatex: separate maxnames for authors and editors

Unfortunately I don't know what style you are using, but for the standard ones you may try to patch 4 macros (I hope I didn't forget any):

\usepackage{xpatch}
\xpretobibmacro{author}{\defcounter{maxnames}{2}}{}{}
\newcommand*{\defeditorcounter}{\defcounter{maxnames}{3}}
\xpretobibmacro{editor}{\defeditorcounter}{}{}
\xpretobibmacro{editor+others}{\defeditorcounter}{}{}
\xpretobibmacro{byeditor+others}{\defeditorcounter}{}{}

Not sure this is the cleanest way though.