There's no such thing as lib1 version 3.x. lib1 is a single version. Every import path is a unique package. If you want to make a new version of lib1, you need to give it a different path. This can be as easy as manually inserting "v2" in the path, like /lib1/v2/ or /v2/lib1/

There's no way to use "go get" to get anything other than the head of the master branch... at least, not by default. If you use a service like http://gopkg.in then you can encode a version in the url/import path which can then redirect to a branch in github, so for example import "gopkg.in/natefinch/lumberjack.v2" will get the v2 branch of github.com/natefinch/lumberjack (gopkg.in does this with some git magic)