The Backup Folder is the third Tab od the Options window. The folder is not set by default.

If you share the data of AS3 with multiple PCs, also the backup folder must be a shared one. The difference with the data folder is that while it's necessary to point to the sared data folder in every single PC, the backup folder, once selected on the first PC, is automatically set in the other PCs, since its path is stored in the global settings of the program.