Im with DP on this, your only credit would be a share of the 50% of the song that constitutes lyrics as a writer. What share of the 50% you get will have to be negotiated with the other 2 "writers".
A song is divided into 2 parts....Music(50%) and Lyrics(50%).
As for the musical side, the same applies. The 50% that constitutes music would be divided up between whoever composed/arranged the music. Session musicians who play some instruments normally get paid per session.
Performers and "band" members in this current age of performing other peoples songs only get paid for their performances. Which leaves many "pop" artists wondering why they are not making any money, while putting on the persona that they are making money, when everything is rented and ultimately is the reason that any money they would have made is eaten up quickly. Artists have to pay for everything...tours, publicity, manufacturing, EVERYTHING.
Only then do they ever see a royalty check.
Saw a great interview with Jazzy Jeff a few years ago talking about this very same issue.