Yes, leave the doors open to prevent build up of CO gas. If you cool it down on electric, then the gas should be able to maintain the current temp. If you want to test the ability to actually cool on gas, then turn it on and let it run for somewhere around 24 hours.
How cool it gets set at 3 depends on ambient temp and whether of not the fridge is in the shade. We set ours to 37 and it drops to 32 at night with the outside temp down to 70 or so. Sitting at a site with the sun hitting the fridge side all day, it goes up to 41.