Advanced Filtering

I'm having trouble finding good documentation on Advanced Filtering. If I'm looking at the below, this will return 1 date, 7 days ago (July 19th). I was wondering, is there a way to manipulate this filter to return multiple days at set intervals. For example, today being the 26th, I would like to return 7, 14, & 21 days ago (July 19th, 12th, 5th only).

{
  "last": {
    "count": 1,
    "offset": 7
  }
}

  Thanks in advance!