In the General tab of the sync settings, where by default you select two sync bases, you can define additional bases by clicking on Add more bases and then on the + button underneath the list.
One-way sync / One-way move
Multiple sources are virtually merged and mirrored to all the destinations.
Files with the same name on different sources but with different modification dates or sizes are considered a conflict, causing an error to be logged. If a file has identical copies on two or more sources and only one of them is modified, to avoid a conflict all the old versions on the other sources would also need to be manually replaced.
A file on the destinations is deleted only if it is not present anymore on any of the sources.
Note: multiple sources are meant to virtually merge files spread between different directories or volumes. If you want to maintain separated copies of the source directories on the destination, it may be a sign that you should use the parent directory as the source, exclude it and then include only the desired subdirectories (see Filters).
A two-way sync always behaves the same, regardless of how many bases it contains. In particular, between syncs, a file may be modified only on one base or it will be impossible to determine which change should be mirrored to the other bases, causing a conflict error to be logged. You can read more in Sync types.