The default location for ArcMap 10.2 custom toolboxes would be
C:\Users\<username>\AppData\Roaming\ESRI\Desktop10.2\ArcToolbox\My Toolboxes
And you should be able to copy them to
C:\Users\<username>\AppData\Roaming\ESRI\Desktop10.3\ArcToolbox\My Toolboxes
If you stored them to an alternate location, you may just need to map the folder. Unless you totally erased the folder, they should still be there. If you did delete the folder, check to see if they are in your recycle bin, or hopefully there is a backup.
EDIT: if they were python Addins, you may need to reinstall them.