They're all nice.
Fave is
Frederick, then
Abel, then
Sullivan, though I really do like
Sullivan, so I feel bad putting it last. :P
The problem with
Abel is that it sounds like 'able'. :/
I like that
Frederick can easily be shortened to either
Fred or
Rick (or
Freddy,
Ricky, etc).
Sullivan sounds like a last name, but I like it as a first as well.
All three names feel distinctly masculine.
Frederick Abel Sullivan would actually be a pretty good combo, yes?