Question data are wrong.
According to ours consideration

The 2nd condition given is wrong it should be two more than the age of other two children...

Let the three grandchildren age be a,b and c
Thus, a is youngest, c is eldest.
c = 3a.........(i)
c = 2+(a+b)..........(ii)
=> 3a = 2+a+b
=> 2a = 2+b
Thus, the ages are.....
a, 2a-2 , 3a.

Two of their difference is 3.
Thus, a- 2 =3 or a+2 =3 or 2a =3
a =5 or a =1 or a =3/2.

Thus ages are, 5,8,15
or , 1,0 3 (not possible). or 3/2, 1, 9/2 (not possible.).
The answer should be 15.