Theodore: distinguished and elegant, I like the nicknames
Teddy and
Theo.
Sebastian: I'm conflicted, since I like the sound and the fact it's a character's name in one of my favourite books, "Brideshead Revisited", but it's mostly associated with Adidas-clad skinheads. I like it in a vacuum, though.
Olivier:
Oliver is much nicer.
I think
Theodore and
Sebastian go well together, but
Olivier is French, and neither
Theodore, nor
Sebastian are used in French-speaking countries.
Ollie is oily and childish,
Theo is great, and Seb is dull.