3. 16 17 15 18 14 19 _ _ 76 78

This sequence appears to be two interleaved sequences.
The first sequence: 16, 15, 14, _ .
The second sequence: 17, 18, 19, _ .
The first sequence is decreasing by 1: 16, 15, 14, 13.
The second sequence is increasing by 1: 17, 18, 19, 20.
The last two numbers 76 and 78 do not fit this pattern.
It is possible that the last two numbers are unrelated or part of a different pattern.
Given the provided numbers, a definitive solution for the blanks before 76 and 78 cannot be determined with certainty based on the interleaved pattern.
The question is incomplete or requires external knowledge.