Act 3 is a long finale with the odd insertion of another country race which really belongs in Act II instead of III, and which interrupts the progress toward the showdown with Dierker. And even that isn't much of a showdown. But getting to the anticlimactic scene atop the Tour Eiffel involves a lot of bullets and very little sneaking. So get ready to fire away.
C'est Ici L'Empire De La Mort
You must find the resistance via a new entrance to the Catacombs, and it's paved with Nazis and very old bones. Fight your way carefully, no sneaking here, until you make it to the big cavern. Watch for Terror flamethrowers and shotguns.
It's a long way past a lot of old bones.
When you reach the cavern, fight your war around to the left, following the glowing blue Resistance symbols until you can jump across to join the others.
Nice of the Resistance to take the time to mark the way with that cool glowing blue paint.
Once you've joined your friends, defend that space until the cut scene. Make sure to kill Santos, the traitor, on the other side along with the other key enemies marked with red triangles, and hope you're not too attached to Luc.
Luc, that has GOT to hurt.
The Kesslers are captured, Luc is dead, Vittore is dead, the world is crumbling and it's up to you to save it. But how about another car race first? Look for the checkered flag on your map and head north. Far north: once again you've got a long trip to get to a mission. The Western HQ garage is still in business, so you can pick up an Aurora there if you wish, but an Altair is waiting if you take a slower car to the race spot. The Aurora makes this race a piece of cake though. Use the Nitrous to get to first place at the start, then keep her there. There's not much of a straight for the boost, but if you hit it right before the start/finish line, it should be effective. After the race, SAVE after the Mission Complete, then load to respawn at Le Havre. You can then take your new toy, the Silver Claw Mk II for the LONG drive down to the very bottom of the map.
I've got to win this race, the fate of the free world depends on it! Oh wait, no it doesn't.
This is your last big shooting match, and it's a doozey. Skylar drops you and Veronique off at the base of Doppelsieg. Grab one of the German vehicles and make a run for it up to the top: there are too many soldiers to fight your way up there and it's far away. Plus, in typical style for characters like this, Veronique is a complete twit in a fight and much more of a liability than a help. So just get her in the car and save her from herself.
It's déjà boom all over again.
Once there, go inside the complex and begin fighting your way backward along the same path you used at the beginning of the game to escape Doppelsieg. You have lots of Terror guards to deal with, so take cover when you need to. Finding Kessler's daughter gets a cut scene, and you continue on alone.
Oh no, flashback!
Find Kessler and let him out. He insists on destroying the cyclotron. Help him flip the switches and then follow his instructions to blow up the power coils in opposing corners. Watch for guards as you move around. When the core is exposed, the Terror machine gun makes quick work of destroying it, no need for grenades.
The first cyclotron had a fairly short period of service.
On the way out, you must protect Kessler by clearing a path. He won't leave the main room until all guards are dead. Then follow him to the exit for the end of he mission.
Angel of Death
This is it, the last mission, sort of. Drive Veronique to the Eiffel Tower, which isn't easy as Paris is in chaos. Don't bother stopping to have your papers checked. When you drop her off, run straight to the tower and take two elevators and some stairs to the top to find Dierker. There's no boss battle, just a single shot to the head after some chatting.
After the credits roll, freeplay continues. As Sean says at the end, he's just getting started. You get your last Mission Complete and are free to continue working on your perks and achievements. HQs are restored, even La Villette and The Belle, and Paris is still occupied by the Nazis. The entire city is in color, though. The only game area blanketed in Nazi doom and gloom is Saarbrucken, which is guarded with a large indestructible gate. The guards are destructible, however, and a trapped car will make short work of them. You can then walk up, hit the button to open the gate, and stroll right into enemy territory.