CAR SEATS DON'T HAVE TO GO BY A WINDOW!!!!
I'm a former Flight Attendant and if you read the link Wendy provided, you'll see that the car seat can't block anyone's access to the aisle. So that means it can be installed also in the center seat of the center section of a "wide body" aircraft (def. one that has two aisles) OR a window seat. You can put two car seats together but you can't have someone, even the parent, sitting between two. Whether they were ffing or rfing was immaterial. No one could sit between them.
There cannot be a car seat between a person and an aisle, if that helps keep it clear.
In theory, you could put it in the center seat is no one is seated by the window. Not sure why anyone would do that. Perhaps a really empty flight and you were flying with someone across the aisle?
Someone mentioned company policy "overriding" FAA rules. Not allowed BUT the airlines can "enhance" FAA rules already in place. I switched airlines and my old one didn't allow blankets over knees for take-off and landing. My new coworkers found this amusing, that those of us from the other airline were telling passengers this.
A F/A from another airline said that a few car seats were allowed on the aisle on, I think it was Delta (an airline I'm not familiar with and only flew once in the 1980's). A few ffing seats were allowed. I was never able to confirm this. Both my airlines were direct, no car seats between passengers and aisles, no exceptions.
Obviously the window is the most logical location but it isn't the only one! This is of course FYI, as WN only has 737's and therefore, only one aisle on all their aircraft. Xmas travel will be busy so the chances of an extra seat are slim.
As far as the OP is concerned, you're a good candidate for CARES harnesses for the older two. Rent them on Ebay or Craigslist.
If you do find yourself taking two or three car seats, I would get the gizmo to carry one on the back and puzzle the other two on one metal luggage cart as suggested earlier. Your carry on, which I recommend only one, use a backpack and hang it on the car seat contraption.
Practice whatever you decide at home first, before heading to the airport. Use extra bungee cords to double secure it.
I wouldn't count on getting a F/A to help with the car seats. WN usually has minimum staffing and they're not allowed to leave their doors unattended so no one will probably be available to follow you down the aisle. What I've done is leave a seat by the door, tell the F/A what I'm doing and then dumping the rest in the seat (including children), wiggling back up to get the seat. When people saw me doing this, either the F/A handed the seat over to me or another passenger with a free hand brought it to me.
Don't "pick a fight" if anything comes up. If you get a F/A with attitude (not hard to do during the Xmas rush), and things are starting to heat up, ask to speak with the head F/A. Don't "get into" it. I do recommend printing up that link to show if there is a misunderstanding. Another trick is to simply ask the F/A to show you in the manual that whatever you're doing is not allowed.
The FAA states clearly that you are allowed to use your car seat for a child who fits it and for whom you've purchased a seat. So that means your right to use it for the older two is guaranteed. You must be reseated if there is a problem with your actual seat(s). If they force you to check either seat, this is a FAA violation and let them know that you will be reporting this to the airline. WN will take this complaint very seriously and you may also threaten to copy it to the FAA.
Remember that the lap baby does not have any of these rights.
I hope it doesn't come to this! Be aware that the CARES harness does not have these restrictions. You may seriously want to look into at least one for your oldest.
Good luck!