3) 16, 17, 15, 18, 14, 19, -, -;

The pattern alternates between adding 1 and subtracting 2, then adding 3 and subtracting 4. The sequence is +1, -2, +3, -4, +5, -6, +7, -8.

The missing numbers are 12 (19-7) and 20 (12+8).