I'm level 30 and  have three party members currently. Nolan, Bridget and Gregory. Am I supposed to have a fourth? I've had about 3 characters join me temporarily. 

Thanks!

As you level up, you should keep checking back with those factions / individuals you have helped for more quests. Most characters who join you temporarily will eventually be able to join your party permanently, so without giving too many spoilers I will say check back with them (or their faction leaders) again.

Depending on how far into the storyline you have progressed, there should be between two and four more characters able to join your party now. You'll just need to talk to three of them to do their next quest / get an idea of how to help them further, while the fourth is a semi-secret character who is available once you have defeated the giant Antlion terrorizing the Deadhanders at the Motel/Checkpoint.

I don't want to give too many unsolicited spoilers here, but if you need additional info let me know!