From my extensive experience with C3 I can suggest trying different procedure when you put it on.
Once the helmet is on your head, try rotating it backwards until the neck roll bottoms out at your neck. This moves the ratchet forward (by just a little bit, but usually it's enough) and it will no longer bother your neck. Also, when you close the chin bar, hold the top of your helmet with one hand to prevent rotating the helmet forward. This procedure works for me.
Originally Posted by PuraVida
Speedz, can you elaborate a bit on this adjustment and how you fixed it? I've tried on several Schuberth helmets, and love them except for this problem. I've talked to a local rep and even emailed the company about it, but neither had a solution. Apparently they're aware that this issue affects a small number of people, but currently they have no plans to fix it.