Ok, first, you're shooting at 75 feet. You don't sight your scope at 75 feet, especially not with a rifle where the scope sits a couple of inches above the barrel. That said, you still are doing something wrong there. 3 mildots are about 10 MOA, or 2.5 inches at 25 yards. If your scope is 2.5 inches above the barrel you're scope line is parallel to the bore. It should be that with the reticle centered. As most scopes have at least 25 MOA travel you should bottom it out, especially not low, because if you're shooting low at 25 you will be shooting low everywhere.
My first suggestion is, take the scope off and remount. Just to make sure you didn't tighten something wrong. Then remount again, but with the rings switched. If the rings are uneven, you just made yourself a great long distance rig as your now have a huge adjustment range
. If that doesn't help, try a different scope. If that can't sight either, the rail on your gun is bad.