Forgive my lack of knowledge but do floor stander 3-ways need or benefit from a subwoofer?
Are the crossover duties handled by the speakers or by the AV amp like bookshelves?
What about when no sub is used?
Are floor standers always set to large?
There are very few speakers that will reach the last octave at power without a sub. The few commercial offerings that do pretty much require a mortgage.
Even then for HT use you need to be able to capture and blend in the LFE channel. This means the speakers need to be active and have their own amplifiers and more than one per speaker.
So unless you have an unusual exotic then you need a sub or two for optimal movie watching.
There are some floor standers that contain a powered subwoofer section. These are not usually well executed.
Not all floor standers have the power reserve to be used full range, but if they do, it is usually best to set them to large and gently supplement them with a sub.
The main advantage of a good tower speaker, and the advantage is considerable, is that you have the power above the 60 to 80 Hz range. Most of the power demands of music is in the 80 Hz to 2.5 KHz region.
Most speakers quickly become excursion limited in the 20 to 40 Hz range and many above that. Using a sub with the speakers set to small off loads the drivers in the main speakers, and that is one of the principal advantages of a sub.