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 on different sources with the same name 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, all the old versions on the other sources would also need to be manually replaced in order to avoid a conflict.
A file on the destinations is removed 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. To maintain the same directory structure on the destination, you should instead select the common parent directory as the source, exclude it and then include only the desired subdirectories (see Filters).
Two-way sync
A two-way sync always behaves the same, regardless the number of bases. 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.