I read that minor canker sores heal without scarring. my 3 mm-wide canker sore seems to have healed three days ago, but i can still see a white line there and redness. is that a scar? or is it going to heal more?

It may well heal!: ... and we hope it heals well! While they are very irritating cancer sore do heal well. The source of the irritation is the virus breaking through and the neuritis in the area of the ulcer. You are young and it would be a good idea to find out what is stressing you so that you can reduce the predisposition to having emotional episodes or spicy foods that cause ulcers.

STILL HEALING: A canker sore can last up to 8-10 days and slowly heal or fade away. No scar will be left but try to reduce stress and acidic foods or liquids to keep the sores from returning.