Do you play the piano?

You can play the parts, build it one note at a time.... in MIDI. Heck, you really don't have to play piano. You can "mouse" the notes in.

Create the parts in a DAW using midi tracks. One for each singer. You can fix and adjust the notes as needed in each track. Simply export the tracks individually as waves or mp3 and send to the proper people.

That is how I would likely do it if I was going to do it...

There's also this>>>

You can also use Melodyne if you have it to create and edit the harmonies in the audio realm.

You can find my music at:
www.herbhartley.com
Add nothing that adds nothing to the music.
You can make excuses or you can make progress but not both.

The magic you are looking for is in the work you are avoiding.