Three partitions may be over-manageing things. Unless there is a requirement that Set A be mounted during the week that you want to write to set C, just create a single partition, and load that week's 5 tapes into it
if you really do need to mount two weeks worth of tapes at a time, go ahead and do that.
In either case, create only a single job using a Media Set that has an OPP of 16 days, and point it at "All Pools" rather than a specific partition
To get the tapes used in slot order, thr first time a set is mounted, re-lable them starting with the low order slot. After that BackupExec should always use the lowest tape number first based on it's least recently used logic
Also set the Global Media Management option to "Use overwritable media in target media set before scratch" rather than the other way around