In GM mode, can a group of three bring along a fourth "npc" player to round out the group like in the campaign?

You can set any npc to follow the party. The npc will travel along the party to any scene you load. You can set him as an ally, so he will fight along the players. Just remember to drag him nearby the players or posses him and walk besides them.

In my campaign, they found a monk willing to help them, so now the party has a healer who help them when someone is missing the game.

That's excellent! I could create a few "hired" guys and place them near where the group might meet up. A couple questions...Can we save created npcs for later use in the GM mode and what happens to the npc when I reload to another map location?
Thanks!

@Rhime.

1. You right click your NPC and choose "Save as". It will be saved under Exported Creatures in the Creatures & Items panel.

2. If you right click your NPC and choose "Follow party". It will always stand beside the characters when you load a new scene. But, as said above, it will not follow inside the scene, so you have to manually move the NPC when the party moves, or possess him and walk him yourself beside the party.

3. I have for some time tried to get Larian or a modder to make it possible to make NPCs actually follow behind a character inside the scene as well. But nothing so far, unfortunately. A modder had successfully made this possible by adding a status effect to a NPC, which made him follow. But he quit and no one have picked up that ball, unfortunately.

That was me. It's sitting in my project folder. Larian broke story scripting in GM mode and I am currently waiting for them to fix it.

We really need the option to create a 4th character someone can control. The NPC's are stupid when in follow party mode.