Explanation:

A and B together=8 days

B and C together=12 days

A, B and C =6 days

1/A +1/B +1/C =1/6 equation 1

1/B  +1/C  =1/12    equation 2

1/A  + 1/B =1/8     equation 3

1/8+1/c =1/6

1/c =1/6  -1/8

1/c =2/48

C=24 days

1/A +1/12 =1/6

1/A  =1/6 -1/12

      =1/12

A=12 days