For my three device, it shows a three logo at start-up and shutdown, and comes with a few preloaded apps.
Mine had stuff like facebook and soundhound on ready for me to use (don't know if that's standard).
Those are standard on the One so not operator bloatware. If there is anything specific to Three then it will probably be in a 'Three' subfolder.
A custom ROM won't unlock the phone BigToe.