It has 5 voices. Take some graph paper and count the bars in the fugue. Now decide on arbitrary height for marking active vs inactive on all three themes. At height 8, mark the bars where countersubject2 is active, and finally at height 4 mark the bars where countersubject3 is active. Now look at the symmetries that show up.
All blocks are either symmetric with respect to their center pattern, or are pyramids, or both.
Moreover, all possible combinations of the digits 1,2,3 can be found in some pattern. The combinatorics discipline was barely developed at the time of Bach as it was mainly developed by Pascal and Fermat. It appears to be that Bach was quite knowledgeable about the basic issues. Nobody still knows if he actually intended those patterns to be included in his music or he just composed and those things just showed up.