I posted a similar question a few days ago and I haven't heard from anyone yet either, but I will tell you what I know. The tenor and the soprano parts are usually the same just at different octaves. The alto part just keeps the melody or balances it out. I usually start by trying to use the 1 (tenor), 5 (alto), then 8 (soprano) ( same as the 1 just and octave higher) of whatever key the song is in. Just play the melody of the song using those notes as a starting point for each part and just tweek it from there.
Hope that helps,
Keysoflife06