Why has my 3rd wisdom tooth grown in so slowly? i'm talking about a year after my first two..

Variability: Lots of variability between different wisdom teeth in same patient. Within normal limits.

Don't play by rules: Wisdom teeth exhibit a tremendous amount of variation in when and how they erupt. It is possible that the tooth is partially impacted and doesn't actually have room to complete eruption. The question is: is it hurting you, does it have a cyst or infection. If the answers are no, then you can just give it more time. If yes than have an evaluation by your dentist.

Varies: The roots may be forming slower than the other teeth, the alignment in the bone may be in a different position than the others, or position and angle of the tooth in front of it may affect.