I want to know the meaning of "3.x " version of koha.

If someone refers to "3.x," they are referring in general to the Koha 3 series of releases, 3.0 through 3.22. Newer versions would be referred to in general as "16.x" since the project moved to date-based versioning numbers. The latest release, v16.11.00 was released in November (11) of 2016 (16). -- Owen -- Web Developer Athens County Public Libraries http://www.myacpl.org