This is the way I usually figure out how much to serve for a large party:
For meat- 1/2 pound per person
For chicken- 2 pieces per person
For baby back ribs- 1/2 a rack per person
Your three chickens will yield about 12 servings
The 2- 7lbs pork butts will yield 30 servings
The 6 racks of ribs will yield 12 servings
Of course, this is just an estimate. You always have to consider that people would want to try a little of each. And having more is better than having too little. It’s better to end up with left over than leaving people wanting more. What I usually do when I end up with too much food, I give everyone a portion of it to take home. This way they’ll enjoy it the next day (and remember the great party), and I don’t up with too much food that goes to waste.
I hope this helps.