Portion Size Three Meat Plate

That will depend on sell price, cost and budget.

Key word is "fundraiser". I've done too many of these. First consideration is most will expect a heaping plate for their donation and forget its to generate money for the group. Based on that I'd usually plan on 3-4 ribs, 6-8 oz brisket and 4-6 oz sausage. Hopefully you are having this served or the hogs will clean you out in a hurry. I hated when the group didn't want to set a price based on fair market prices. Some wanted a freewill donation then bitched that they hardly covered food cost. I had to empathize it's a fund raiser not a friend maker and portion control was a must.

What are you paying for the meats? It's a simple math problem really if you know whatb you want to spend.
If a 10 bone rack costs $10, then there's $1 per rib. If sausage costs you $3 per lb, then every 5oz link costs you $1. If brisket costs $4 per lb, then every oz of brisket will cost .50. If it's a fundraiser, have you reached out to butcher shops or grocery stores for donations?