As you already noticed, the VMS sort expects fields with a fixed start position within a record. You can not specify a field by a separator. If you want to use the VMS sort you have to make sure your third field starts at the same column, for all records. In other words, you have to pad preceding fields. If you have control on how the file is created, this may work for you. If you don't or you don't know how big the string in front of the sort field will be, this may not be a workaround. Maybe changing the order of the fields is an option.
On the other hand, you may find GNV installed on your system. Then you can try to use its sort, which is a GNU style sort. That is, $ mcr gnv$gnu:[bin]sort -t, -k3 -r x.txt
may get you the wanted results.