Original: The first beta for Windows 2000 was released in September 1997 and several further betas followed until Beta 3 which was released on 29 April 1999.
Simplified: The first software test version (also known as a beta version) for Windows 2000 was released on September 27, 1997. Several more test versions were released until Beta 3, which was released on April 29, 1999.