All of them have different purposes:
Choose Pursuit if you intend to use it to counter Psychic types like Lati@s to prevent switching. This is better suited for tank/support.
Choose Knock Off if you're using it as a lead, paired with Stealth Rock. This is more for support.
Use Crunch if you're running Choice Band/ Scarf or Dragon Dance. This is for sweeping.
Based on your team's requirement, you can opt for any of the three.
Edit: As sumwun pointed out, T-tar can't learn Knock Off, so that's off the table. I didn't cross check and merely gave what I felt was the most appropriate answer. That's totally my bad. For future reference though, for any Pokemon that can learn all three, this answer is still valid (except Stealth Rock of course) for their functionality.
