195524

Here is Jake's description of how he figured out the answer
1) I wrote a number line and crossed out 3
2) The three consecutive numbers in the 4-digit number cannot be 0, 1 and 2 if the third digit of this number is the sum of two of them, and each digit is only used once (0 + 1 = 1, 1 + 2 = 3)
3) For the third digit to be a single number under 10 and the sum of two consecutive numbers above three, it must be nine and the sum of 4 plus 5.
4) The three consecutive numbers in the 4-digit number must be 4, 5 and 6
5) The 5-digit number must contain the 5 numbers left over - 0, 1, 2, 7 and 8. Only 7 or 8 can go in position 2 and 4 of the 5 digit number
6) I used trial an error to find the ones digit of the 4-digit number. Only 4 5 or 6 could go there. If 5 went there, the ones digit of the answer would be 5 (5 x 3 = 15) - not possible as I can only use each digit once and only 0 1 or 2 can go in this spot. If 6 was the ones digit of the 4-digit number, the ones digit of the answer would be 8 (6 x 3 = 18), but I know 8 can't go there. So the ones digit of the 4 digit number must be 4, making the ones digit of the answer 2 (4 x 3 = 12). I know that the tens digit of the 4-digit number is 9. 9 x 3 = 27, plus the one carried over from the 12 = 28. So 8 is the forth digit of the answer.
8) This makes 7 the second digit of the answer as it can only be 8 or 7
9) I carried on using trial and error to put the 5 and 6 in place in the 4 digit number so that the answer was correct and made up of only the numbers that I had left
10) I ended up with 5694 x 3 = 17082