[KnucklHed BBQ]

Honestly brother, unless you've done this sorta thang MANY times before, I'd REALLY recommend dropping the meat count to 2 MAX. And take the 2 that you're most comfortable/consistent with.
The timing alone of having brisket ready and great turkey is tough (pork should be a gimmie if you're even considering doing the job) and then having to grill off chicken too... It's a lot man. And carving, i like to have carving station for brisket for the best experience, but turkey prolly means you need to carve that too...

BUT - your question was about pricing - first off, for 2 meats (Lets assume brisky & pork), I plan on everyone (mostly) taking some of each.
Brisket - Its EASY to serve 1/4lb real quick, 2 slices roughly.
Pork - a bit easier to go 2-3oz

The way I price 2 meats for catering, I charge for the most expensive meat and add a per person charge for the second less expensive meat.
For instance:
Brisket + 3 sides = $17.95/pp (approx 40% loss from trim and cook)
Add smoked pork (Inc buns, sauces etc) $3/pp

This calc is based on a bit less brisket needed since there's pork also available, and not as much pork as usual since there's also brisket... Essentially my view on it is that I'm getting menu price for brisket and the pork is an easy add on that essentially bumps the overall per person cost to $21/pp without not much more work.

Crush it man!