This is an open question. 1, 3,5 has many answers.
(), 9, (), 12, (), (15) Note: The difference between 2, 4 and 6 is 3 respectively.
(8), 9, (1 1), 12, (14), () Note: 1, 3, 5 differ by 3 respectively.
Or: (2), 9, (4), 12, (6), () Note: 1, 3, 5 differ by 2 respectively, not necessarily in a certain order.