During the Character Creator 3 installation, the installer will auto copy morph sliders from the Character Creator 2 Template / Custom folder into the corresponding Character Creator 3 folder location. The morph sliders will become Character Creator 3 sliders once you launch the Character Creator 3 program.

Other content type like Cloth, Hair, Project, and others, can be manually dragged from the Character Creator 2 folder --or--
manually copied from previous Character Creator versions into the corresponding Character Creator 3 folder location. In order to avoid any version mismatch, please DO NOT replace the existing folder and files with the same name. Only copy the specified content that you wish to use, instead of copying the entire root folder. May we suggest that you use a file copying utility like: http://www.tgrmn.com/.

From Character Creator 2 for iClone : -
C:\Users\Public\Documents\Reallusion\Template\Character Creator 2 for iClone Template
C:\Users\Public\Documents\Reallusion\Custom\Character Creator 2 for iClone Custom

To Character Creator 3 for iClone : -
C:\Users\Public\Documents\Reallusion\Template\Character Creator 3 Template
C:\Users\Public\Documents\Reallusion\Custom\Character Creator 3 Custom