Question 242371
Let the girls ages be A, B, and C.
{{{B=A+1}}}
{{{C=A+2}}}
The youngest is A. 
The sum of their ages is 
{{{A+B+C=3A+3}}}
{{{3A+3>39-A}}}
{{{4A>36}}}
{{{A>9}}}
10, 11, and 12.