I don't believe you'll find a clone of The Semware Editor, and especially not on all three operating systems.
Your best bet would be to find a customizable text editor available on the three operating systems, and customize it to emulate The Semware Editor as closely as possible. This would probably be a very hard and long job. I would really advise you to find and get used to a new text editor.
One candidate would be XEmacs (although it does require Cygwin under Windows):
XEmacs is a highly customizable open
source text editor and application
development system. It is protected
under the GNU Public License and
related to other versions of Emacs, in
particular GNU Emacs. Its emphasis is
on modern graphical user interface
support and an open software
development model, similar to Linux.
XEmacs has an active development
community numbering in the hundreds,
and runs on Windows 95 and NT, Linux
and nearly every other version of Unix
in existence. Support for XEmacs has
been supplied by Sun Microsystems,
University of Illinois, Lucid,
Amdahl Corporation, BeOpen, and
others, as well as the unpaid time of
a great number of individual