Emacs supports macOS natively. See the file nextstep/INSTALL in the distribution.
There is a separate FAQ for Emacs on MS-Windows, see FAQ for Emacs on MS Windows.
Emacs supports GNUstep natively. See the file nextstep/INSTALL in the distribution.
To build Emacs from source for MS-DOS, see the instructions in the file msdos/INSTALL in the distribution. The DOS port builds and runs on plain DOS, and also on all versions of MS-Windows from version 3.X onwards, including Windows XP and Vista. Pre-built binaries may be available at https://www.delorie.com/pub/djgpp/current/v2gnu/emacs.README
For a list of other implementations of Emacs (and Emacs look-alikes), consult the list of “Emacs implementations and literature,” available at
https://www.finseth.com/emacs.html
Note that while many of these programs look similar to Emacs, they often lack certain features, such as the Emacs Lisp extension language.