The sequence appears to be the factors of 84. The complete list of factors for 84 includes:
1, 2, 3, 4, 6, 7, 12, 14, 21, 28, 42, 84.
So the missing factors are:
3 for the first blank, 7 for the second blank, and 14 for the third blank.
The fully filled sequence would be:
{1, 2, 3, 4, 6, 7, 12, 14, 21, 28, 42, 84}.