To tackle the problem of making 3 people sound like lots of people I'd recommend this.
You will record two separate takes of the vocal but have the singers standing behind each other, so different distances from the mic. We have person A, B and C.
track 1 - B, C, A
track 2 - C, A, B
Then pan those hard L and R out to speakers that are even further back from the singers. Now record track 3 - A, B, C, L speaker, R speaker. Track 3 can actually become tracks 3(left) and 4(right). It depends if you want the "gang vocals" to be a mono or stereo source, lead or backing, etc.