Hi there! Curious as to what's on your mind regarding each item. I have my own preferences, but will share after you.
Hello, well I really need food. Have a lot of extra people.
Awesome - same, I also need a ton of food. Will firewood be of interest to you as well? I'll be happy to compensate in this way
Firewood is not overly interesting to me, but water is somewhat enticing.
Ok, great - I can give you all of my waters in exchange for a majority of the food and half of firewood
Half of firewood? There are 3 so one of us has to get more. I would need more of that. 
Yep, I know there's three and no way to split evenly lol - can you give me a justifiable reason as to why you deserve more over me? Please convince me.
Well, food is the most important and you're getting that so I'm gonna need more of the others.
What are your thoughts on the amount of water you need?
I would need 2 out of 3 as well.
Interesting - you're essentially saying that you need the majority of two of the three items. Hmm...
Yes, thats all. I need 1/3 food, 2/3 water, and 2/3 firewood.
Still not convinced in any way that you need more items in total over me. Please clarify!
Food is what I prioritize much more than the others. If you want the majority of the others, I will need all the food.
Ok, I'm willing to compromise to speed this up - I'll go with the original plan you've outlined, and I'll take 2/3 food, 1/3 water, and 1/3 firewood