Unless you're using a rack mount processor for thickening or simple harmonies (unison, octave, 5th), you'll have problems with a melodic 3rd, either major or minor. We use the TC Harmony G-XT to add a 3rd harmony to mine and one other singer, and usually the mix is set around 1/3, so that you barely hear it but adds to the mix without being overbearing. The G-XT has a guitar thru so it can figure out the major/minor third based on the chords of the song, which makes a big difference. I've also got footswitch control, so that I can turn it on and off when needed - it's a lot easier for me to know when I want that harmonic third vs. an engineer, unless the engineer is always with the band and knows the songs by heart - and isn't doing the normal combat audio tweaking other parts of the mix, feedback, etc.
I'd keep it simple, and have the musicians control when it's being used, and adjust the mix ahead of time to use sparingly.