The software features, like many other, a Help section from where you can get various details about the software (installed modules, update status, version info, etc). To obtain the version of your installed Mach3 application, go to Help > About.
Additionally, go to Start and type Programs then select Programs and Features. Locate Mach3 in the list and next to it will be the version of the program.
Additionally, you can right click the executable file and switch to Properties > Details.