I am most often in male mode. Even when typing here. Not just attire, (but being 'dressed' helps), but there is a softer side that is unexplainable when 'en femme'.
When I speak in third person it is usually because my male side is unsure of something that I do while en femme.
An example is shopping. My male side HATES it. He is a tightwad. When he types, "Barbara has a shopping problem" he is both correct and in control of the rudder at that moment.
When I, Barbara, type in first person it is because I am feeling secure and open about my femme side. I can answer questions respond and it feels sincere and not contrived.
When he is uncertain about why it is he wears a dress at times, he refers to Barbara in third person.
This is a GREAT question, and answering it has been illuminating, I just realized that I am Barbara as I type this.
I think I handle feelings better as Barbara..
Edit: I just read Sher's older thread on this. I was STRIKING who chimed in that they don't.
I don't want to speak for anyone else, but Third Person seems more common with "Crossdressers" like me especially if we for the most part happily present in our birth gender most of the time.
Those that I would see (as an outsider) as truly "Transsexual" meaning for me they ARE the opposite gender to their apparent birth gender, don't seem to do this. If they are male, they are male so no matter what they wear they are him. If they are female, they are her regardless of clothes and environment. This held true for those that circumstances permit living their true gender 24/7 and those who would like to but cannot.
It is only those of us that think of ourselves as both genders or primarily one gender with a costume change that have this trait.
I now think it is a bit of a 'tell' for where we are on the ts/cd continuum.