This year when we were in Alaska we stayed for 8 days in Anchorage. I unloaded the camper the day we got there till the day before we left. I did lower it down as low as it would go and also kept the front a little higher than the rear. We slept in it every night with no problem.
But as said some camper are not meant to be used off the truck so you need to check if yours is capable of it. As mentioned about some campers do not have enough flooring to support people inside of the camper off the truck.
Also welcome to the RV.net. Ask questions, almost all questions have been answered at one time or another but there are a lot of good people on here and they well try to help you any way they can so ask away.
If you only have 3 jacks then I would never use it without some support under it. They are very capable of tipping over if you go to the front on the side of the single jack.