In this case I mean Fleet ship T3. That includes upgraded versions of the Defiant, Nebula, Nova, Excelsior, Ambassador and Mogai classes.
Technically the ships are not seperated by tiers, they all come with 10% more hull and shields and a tenth console slot. Some have alternate boff layouts with the exceptions of normal T5/C-Store baseline ships.
The "tier" in this case just means the shipyard tier that unlocks them.