Really if you want to run a 15,000 btu unit and anything else at the same time you should go with the biggest generator you can afford. It will be more efficient. 5000 W + would be better. 3500 barely meets the needs of starting it
I have The EU3000iS and it will run for up to 20 hours on a single tank of 3.4 gals of gas but only if it's not running a real heavy load.
You can get the smaller 2000 versions and connect two of them together but they only hold 1 gallon and run up to 8 hours. That means you might not make it through an entire night before needing to refill.
At this point I wish I would have got the 7000.
I found a 13,500 BTU Coleman air conditioner called a Power saver that runs on 10 amps that will work fine for my generator.