Catering event for BBQ for 30 people.
When you visit your local BBQ places you normally see a few standard options.

For the pulled pork and brisket, it is usually sold by weight. I believe a 1/2 Pound portion is the most common from my experience of eating BBQ with friends.  

For ribs, they are normally sold by half rack or full rack.  A full rack is a lot of food so I would not think anyone would expect to eat that at a BBQ. You would probably be good with 5 or 6 ribs per guest.

These amounts of meat per person along with some popular BBQ side dishes in your area should provide plenty of food for your guests.

The big part is figuring out who wants what meat.  If you already know then you can easily plan for that.  If you don't know you can do enough for 10 of each and let your guests fight over who gets what.  If it was me, I would probably make enough of each meat for 15 people.  That should assure nobody goes home hungry and you will probably have enough left overs for a few days.