I have a Southwest Airlines domestic flight landing in Las Vegas at 4:40 pm in Terminal 1. I have an international flight on Norwegian Air. leaving from Terminal 3 at 6 pm. Will I have enough time to pick up my bag and make my second flight?
This is not enough time, certainly not with checked luggage. Check-in, including bag drop closes 1 hour before departure. That's 5pm, 20 minutes after your flight is scheduled to arrive, and you must collect your bags and change terminals, which, as I understand it, requires a bus that comes every 12-15 minutes. You also have to get yourself out of Terminal 1, which is a large complex of several buildings, and wait for your baggage to arrive, which has been known to take a while
If you arrive 10 minutes early, your bags arrive quickly and are the first ones off the baggage carousel, the curb where the bus pulls up is right in front of you, the bus leaves the moment you get there, and the Norwegian check-in desk is right in front of the door, and you run like heck, it's not, you know, utterly impossible. If any one of those things doesn't happen, it's not enough time. If your Southwest flight is 10 minutes late and it takes 10 minutes for the bags and you to make it to baggage claim, you're already too late before you've even left Terminal 1.
And since you've arranged your own connection by booking two separate tickets, the airline isn't responsible if you miss your flight, and you could well be spending one or more nights in Vegas. I'd book a much earlier flight.