Looking at the model disc that you have I think once you adjust the front downward you will be discing in the center. Appears that the rear gangs have a gap between them so if the front isn't low enough you're not getting any contact in the center. Just going by the picture.
Going a little faster with the tractor speed will smooth it out more than go really slow. Should have some type of 3pt depth control on the tractor controls. Remember to lift the disc when you take turns or you'll take out the bearing in the disc and maybe damage the bottom arms on the 3pt...
Should be able to adjust the top link on the 3pt by turning it. This will let you get the front down farther and then it should be making contact with the ground. If you have just one side of the disk digging in way more than the other you can adjust the lower arms individually to get it level...
The gangs are adjustable on most discs. What type/brand of disc are you using? Pull behind? 3pt? Setting the discs as straight as possible is the least aggressive. I went with the straightest setting on my 3pt disc the last few years. It's not as hard on the equipment and I have mostly been...