3rd mission?

On my map, I have two swords through The Great Northern Wall, and two swords through Fort Ranik. Where is the next mission? I though each mission leads you to the next one.

The next one starts at the ruins of Surmia.

The missions won't always lead you right to the next one. Sometimes they drop you at an outpost or city and you have to walk through an exploriable area to get to the next mission. There should be an NPC in the place it drops you that will give you a quest telling you where to go.

In your case, you have to walk out of the outpost to get into the explorable area, go through the opening in the wall nearby (there's a collector next to it), then make your way Northwest. Its a fairly short walk, so you shouldn't get lost.

When I leave the outpost, I enter Regent Valley. I don't see any opening of a wall nearby. In Fort Ranik, there is no NPC with a quest for me. Is Regent Valley the right map to get to the Ruins?

you must be at fort ranik still, no, the frontier gate is where you should be, you leave that and it takes you to the eastern frontier. Follow the path as it leads to the left and that will take you to the ruins of surmia.