Why is this relation in 3NF?
The mistake is in your understanding of transitive dependency. From wikipedia: Transitive dependency


  In mathematics, a transitive dependency is a functional dependency which holds by virtue of transitivity. A transitive dependency can occur only in a relation that has three or more attributes. Let A, B, and C designate three distinct attributes (or distinct collections of attributes) in the relation. Suppose all three of the following conditions hold:


1. A → B
2. It is not the case that B → A
3. B → C



  Then the functional dependency A → C (which follows from 1 and 3 by the axiom of transitivity) is a transitive dependency.


In your case though, (2) does not hold:

1. U → T                                  -- correct
2. It is not the case that T → U          -- wrong
3. T → V                                  -- correct


Therefore {V} is NOT transitively dependent on {U}.