The three possible 3-digit numbers are:
311
- the first digit "3" is thrice the last digit "1"
- the sum of last two digits, 1+1, is 2.
- 2 is one less than the first digit, 3-1 = 2
632
- "6" is thrice the last digit "2"
- the sum of last two digits, 3 + 2, is 5.
- 5 is one less than 6, 6-1 = 5
953
- "9" is thrice the last digit "3"
- the sum of last two digits, 5 + 3, is 8.
- 8 is one less than the first digit, 9-1 = 8
(Note: Do you see the pattern? :-)