Specific question about Arrow's Impossibility Theorem

We suppose
N (Non-dictatorship),
P (Pareto Principle),
U (Unrestricted Domain) and
I (Independence of irrelevant alternatives)

in a model with 3 individuals and 3 choices (a,b,c) where one individual is almost decisive for [a>b] and Pareto Principle concludes [b>c] (since all individuals rank b>c) transitivity obviously decided [a>c].

Now my prof says due to the Criteria "I" a change of b<c wont change that [a>c].
But isn't b>c as stated in the beginning the requirement for Pareto Principle [b>c] which then leads to the transitivity result [a>c]?

If we change preferences of two individuals from b>c to b<c the pareto principle doesnt hold since just 1 of 3 individuals say b>c. Therefore b<c should be the result and transitivity no longer leads to a>c. it could also be a<c.

What am I getting wrong here?