Welcome to the forum!
This is a great question! I think everyone would probably give you a different answer to this question as there are generally no specifically defined criteria for tier 2 or 3 that make it a completely black and white differentiation.

That being said, I think the best way to conceptualize the move or differentiation of tier 2 and 3 is to think of them in terms of the degree and extent of individualized and customized interventions being used.

It might make most sense to compare in a list style:

Tier 2:
- moderately intensive interventions, like CICO (Check In Check Out) for example
- interventions focused on individuals and small groups
- interventions are fairly standardized and utilized with students in the same manner using the same method
- Some individualized and customized changes made to interventions to suit each student's needs

Tier 3:
- Highly intensive interventions, like a custom behavior intervention plan
- interventions focused on individual students
- interventions are highly customized and utilized in specific ways using specific methods for each student
- Interventions are highly customized and individualized based specifically on each student's needs, behaviors, challenges, circumstances, etc.

Hopefully someone else jumps in here with their thoughts too. Hopefully this adds at least some clarity to your current understanding and perceptions of the tiers.