Re:
Depends, does the 3mm slit go all the way through the tyre?
You can repair the tyre by putting a patch on the inside of it, or using a piece of old inner tube to patch it.
Use your own discretion but err on the side of caution.
Personally, without seeing the cut, I'd be inclined to just replace the tyre.
What model is it? Durano? Kojak?