T,E,T,T,F,
     F,M,A,M,J,
     A,D,G,J,M,

The answers are below.

Treat each line as a seperate problem.

     T,E,T,T,F stands for ten, eleven, twelve, thirteen,
        and fourteen.  So the answer is "F" for fifteen.

     F,M,A,M,J stands for February, March, April, May,
        and June.  So the answer is "J" for July.

     A,D,G,J,M stands for first letter on the numbers
        2,3,4,5, and 6 on your touch-tone phone.  So the
        answer is "P" the first letter on the 7 button.