The two most likely causes are:
1) You're looking in the wrong place (or just not seeing them).
2) Map needs to re-load.
For Item 1:
Hirge appears right next to the street alter in New Garand. She's hard to miss. However, after completing Three vs. Eleven, Hirge will appear INSIDE the New Garand temple.
Griss appears new the well in Kingsbridge. Depending on her amour, she might look like a town NPC.
Adan appears in New Garand in the north west corner, near Trader Pink. He can sometimes wander a little bit and get partial obstructed by a roof of a house.
For Item 2:
If you visit a map that spawns a Companion, than dissmiss that Companion, the Companion won't spawn until the next time the map re-loads (stay at inn, wait long enough, load a quick save from another map, etc).
For example, let's say you have Griss in your party. You run through Kingsbridge on your way to New Garand. You get to New Garand, and dismiss Griss. You pick up Adaon, run back to Kingsbridge to talk to the Wizard to start the quest A Mad Wizard. You start the quest and dismiss Adaon. However, Griss hasn't appeared at Kingsbirdge yey. The map is still active from your trip through it a few minutes before. You run into New Garand to grab Adaon again, but he's not there. The map is still active.
Easy solution is just to head to an outside map (New Garand farmlands etc). Do a quicksave, than load that quick save. That should reload all the maps you're not currently on.
If you have house, your Companions will be inside it, too. It has it's own map loading so it gives you one more spot to swap out Companions.