I was wondering, what is the difference between
LaTeX3. Are the 2 higher versions available for use? I use LaTeX. How can I use the other 2 on Windows 7?
LaTeX2e is the latest stable release of LaTeX. When people say LaTeX they usually mean LaTeX2e, not an earlier version.
LaTeX3 is a massive refactoring of the LaTeX2e code base, which is currently in development. Some of the developers are frequent contributors here and will probably chime in.
The difference to end users between LaTeX2e and LaTeX3 will be many more functionalities included "out of the box" rather than requiring packages.
The programmer's interface to LaTeX3 for class and package writers will include lots of enhancements like built-in data structures and methods to control expansion without so many
LaTeX2e has been the current version since 1994. Every current TeX distribution supports it. LaTeX, I guess you mean LaTeX 2.09, is outdated.
LaTeX3 is a version in development, have a look at