If you rename the filename ... for example ... using replace ... from ' - ' to '; - ' ...
from: Artist1; Artist2 - Title [Artist3].mp3
to: Artist1; Artist2; - Title [Artist3].mp3
... then you can apply ...
Convert: "Filename - Tag"
Formatstring: %artist% - %title% [%artist%]
... and the result will be ...
TITLE=Title
ARTIST=Artist1; Artist2; Artist3