That's impossible, because the older any two people get, the closer the ratio of
their ages gets to 1.  Their present ratio 46/18 is about 2.6, so the ratio
will only get closer to 1, and will never be 3. 

So the question should be corrected to read:

Craig's age y years ago = 18-y
Father's age y years ago = 46-y

46-y = 3(18-y)

46-y = 54-3y
  2y = 8
   y = 4

So 4 years ago, Craig was 14 and his father was 46-4 = 42.

Edwin