Question 907853
You're not taking into account different combinations.
I'm assuming the problem is something like given 3 students. 
a) What's the probability two were born in July?
b) What's the probability one was born in July?
Both questions have the same issue. 
When you look at the possible outcomes. 
J-Born in July
N-Not born in July
JJN
JNJ
NJJ
There are three possible outcomes where two are born in July, you calculated only one. The probabilities are identical for each case.
{{{P=(1/12)(1/12)(11/12)+(1/12)(11/12)(1/12)+(11/12)(1/12)(1/12)=33/1728=11/576}}}
Same issue for the second part. Notice that the book solution is 3 times your solution.