Since it's numb in the middle, it's unlikely that one of the mental nerves was damaged as this would cause total numbness on one side. This is probably just the normal post op numbness that will eventually resolve. Can take quite a while and is variable. I wouldn't worry at this point
After 3 months you should be more toward the normal recovery time. Best to see your surgeon for examination in person.
Temporary numbness after a chin implant is common and most of the time resolves in weeks and sometimes months. Permanent numbness is less common but is a potential side effect. If there is numbness after the procedure, check with your surgeon on chin implant placement to determine appropriate placement.
Sometimes doing Chin implant surgery the nerve that gives you feeling to your chin is stretched or bruised. The feeling should return in 6 months. Sometimes the feeling never completely returns. This is a potential risk of the surgery.