It's a great question, because all three airports are completely different in how they operate when it comes to rental cars and even to some extent hotels.
First, rental cars. They all have one thing in common for rental cars - there are no "in terminal" rental car locations (although SFO is close).
Los Angeles uses the older style bus-to-the-rental-lot concept. You stand on the curb and flag down the bus going to whichever rental car company you're renting from when it comes along. Depending on the company, the time of day, and how lucky you are you might wait for 10 seconds, or you might wait for 10 minutes.
Las Vegas has a large "Rental Car Center", which is located a shuttle bus ride away from the airport. However unlike Los Angeles, it's a single bus for all rental companies. Rather than a curb-side pickup there is a dedicated stop at the airport where there are always buses waiting, and they normally leave every few minutes. It is however an approximately 7 minute ride to the rental car center.
San Francisco also has a single rental car center, however it's located at the airport, and is accessible via the free airtrain that runs between airport terminals. The trains come every 1-2 minutes, and the ride to the rental car lot only takes a few minutes (depending on which terminal you get on at).
So between the three, San Francisco wins as being the more convenient, especially if you have luggage as you can take luggage carts all the way to your car! Las Vegas is a distant second, with LAX being a close third.
Hotels next. It depends what you're after. Los Angeles and San Francisco have a much higher number of "airport hotels" which are very close to the airport. If your simple goal is to get to a hotel and recover from your flight, then Los Angeles probably wins, with San Francisco a very close second (only because there's slightly less hotels, and they can be a little more expensive).
If you want to get to a "downtown" hotel, then Vegas will come out in-front, as the airport is much closer to downtown/the strip than the other two. San Francisco is probably second only because you can optionally skip the car and catch public transport there (BART), but that's only useful if you're not picking up a car at the time. If you are headed "downtown", keep in mind that parking in both San Francisco and Los Angeles is far from cheap - whilst in Vegas it's free almost everywhere!
Now, time to think out of the box!
The problem with renting cars from any of the 3 airports above is that they all tag on a fairly sizable fee for picking up the car from the airport - and it's normally a per-day fee so if you've got the car for a while it can add up.
San Francisco and Los Angeles in particular have a number of hotels near the airport that have free shuttles to/from the airport, and have on-site or near-by rental car companies. For example, the LAX Hilton has a Hertz office inside, and there are numerous hotels with airport shuttles that are within short walking distance of the Hertz and Avis offices in South San Francisco (plus Hertz, Enterprise and probably others will both "pick you up" from a nearby hotel if needed). Especially if you are planning on going straight to the hotel, then catching the free shuttle to the hotel and then picking up your rental car the next morning from an off-airport location can save you a lot of money.
Vegas is a little different if only because there are no free hotel shuttles, so you'll need to catch a taxi/paid shuttle to the hotel - however otherwise it's the same. Depending on what you're planning to do in Vegas you might even want to go the first few days without having a car at all, and then you can rent one from any number of hotels along the strip that have rental agencies inside - saving both the airport fee and a few days rental.
Keep in mind that rates can vary between airport and non-airport locations, so make sure you check both to get an idea of what's available!