Using the digits $$1$$, $$3$$, and $$9$$, we can form~\uline{~~~~~~~~~~}~different 3-digit numbers without repeating digits. 
Overseas Competition->Knowledge Point->Counting Modules->Permutations and Combinations->Combinations
Three-digit:$$139$$, $$193$$, $$319$$, $$391$$ , $$913$$, $$931$$ for a total of $$6$$. 