What makes you think that it isn't possible to run them without the TSO?
They were built with 3 cars, but every modification since has been made on the basis that they are permanent 4 car sets. Changes have been made to the TMS, then there's the digi mods and other bits and pieces. Not to mention the confusion that having a single 3-car 333 would cause and the fact that there's enough slack in the electric fleet that it doesn't particularly matter if a unit is long term stopped.
It would take quite a lot of work to get one out in 3 car form, and even then it's only useful if there happens to be some kind of defect on only the TSO that can't be fixed in any reasonable timeframe (such as major body damage) as most things can be fixed quite easily. The chances of that particular set of circumstances are slim.
So in practical terms, no, you'll never see one running in 3 car form.