29.3.3 Zu dritt, zu viert, zu fünft, zu sechst

This -t does not justify a declension. The morpheme -t is an expression and with cardinal numbers it means
of us/them
.

Zu dritt, zu viert, zu fünft, zu sechst  
  zu zweit = two of us / them
  zu dritt = three of us / them
  zu viert = four of us / them
  zu fünft = five of us / them
  zu sechst = six of us / them

Let's have a look at some examples  
  Sie gingen zu dritt zum Bürgermeisteramt.
= Three of them went to the mayor's office.
  Sie machten es zu viert.
= They were four of them who did it.






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