We just need to set two equations.

We can set the age of the father to X and set the age of the first child to a, and set the age of the secend child to b。So the sum of the age of the two children is a+b. 

so，from the title we know :X=3*(a+b);

and five years later,each of the age of the father,the first child and the second child is X+5,a+5 and b+5.So the sum of age of the two children is (a+5)+(b+5).So we can also get:

X+5=2*((a+5)+(b+5));

Let's get the two equations togher:

X=3*(a+b);

X+5=2*((a+5)+(b+5))=2*(a+b)+20.     X=2*(a+b)+15;

So,3*(a+b)=2*(a+b)+15;

We can get a+b=15,so X=3*15=45.

So the answer is 45.

Hope you like it!