Let

x = the number of days that A can do the job alone

y = the number of days that B can do the job alone

z = the number of days that C can do the job alone

In 1 day A and B will do 1/x + 1/y = 1/9 of the job

In 1 day A and C will do 1/x + 1/z = 1/8 of the job

In 1 day B and C will do 1/y + 1/z = 1/12 of the job

Lets solve the system of equations

1/x + 1/y = 1/9

1/x + 1/z = 1/8

1/y + 1/z = 1/12

we find that

x = 144/11 = 13.0909 days approximately

y= 144/5 = 28.8 days

z = 144/7 = 20.57 days approximately

A needs 13.0909 days approximately to finish the job alone.

B needs 28.8 days to finish the job alone.

C needs 20.57 days approximately to finish the job alone.