Hello, I am new to the forum but I have had my Intex 16'x42" pool for 4 years now. I too "thought" I had the area level on my 1st setup. Hahaha. Boy was I wrong.
My suggestion (which you will not like at 1st) is drain it completely. It sucks, I know, but drain it completely. That 3" difference is alot of water and with stress from users can cause a collapse and injuries. Now I am not an alarmist and I am not saying the sky is falling but hey 1 gallon of water weighs over 8 lbs...To level mine correctly before my 2nd attempt 4 years ago and every year since, I find the center and use a "straight" 2"x4" 10' long with a pivot hole drilled about 2 inches from one end. I sink a piece of allthread in the center.
That is in the center and I have attached 3 levels to it, with a 4' piece of string on the other end. I walk the circle again and again and again and again using a flat shovel to get those high and low areas right. Just my suggestion and yeah it sucks to waste all that water but I sleep just fine knowing I level it correctly every year. Good luck.