I installed a 15 foot above ground pool and the north side is 3 inches lower than the south side. Is this okay and safe to use or do I really have to start all over?

Sorry to get to you so late and I hope you have not filled the pool yet. It should be level and 3" is pushing beyond the limits. Lower the soil under the south side, I'd use a water level. YouTube - ‪How to Build a Water Level‬‏