1.  This phrase of the condition

        "The combined age of the three last year was ¾ their combined ages at present."

    means that the PRESENT combined age of the three is 12.


    Indeed, the combined age of the three THIS year is by 3 yaers more than the combined age of the three at the prior year.
    Hence, 3 years is 1/4 of their present combined age.


2.  So, we have this: 

    A = B + C         (1)
    C = B + 2         (2)
    A + B + C = 12    (3)

    ====>  choose C as the major unknown and express other unknowns via C:

       from (2) B = C-2;                           (4)

       from (1) A = B + C = (C-2) + C = 2C - 2.    (5)

    Now substitute (4) and (5) into (3):

    (2C-2) + (C-2) + C = 12  ====>  4C - 4 = 12  ====>  4C = 12+4 = 16  ====>  C =  = 4.


    So, C is 4 years old.  Then  B = C-2 = 2 years old.

    Hence, A is B + C = 4 + 2 = 6 years old.

Answer.  A is 6 years, B is 2 years, C is 4 years old.