Welcome to the new Club!
The base plate under the nut looks like it's been forced, it should be really flat. Someone must have adjusted the nut with some brute force, because they did not understand the way this works.
FIRST the bolt in the middle must be turned clockwise, because it is locking the nut upwards. Now you can turn the two outside bolts to adjust the height. And you finish by turning the middle bolt counter-clockwise, so the nut is locked into position again.