A group of k elements can be chosen from a group of n elements in [tex]\frac{n!}{k!(n-k)!}[/tex] ways.
[tex](^6 _3)=\frac{6!}{3!(6-3)!}=\frac{6!}{3! \times 3!}=\frac{3! \times 4 \times 5 \times 6}{3! \times 6}=4 \times 5=20[/tex]
There are 20 different 3-member groups.
