how to shift cells down in specified columns?

I think you should have onEdit function somewhere there and this is where you could specify a condition to check only for changes in a specific column:

function onEdit(e) {
  if (e.range.getColumn() === 3) {
      var ss = e.source; 
      var sheet = ss.getSheets()[0];
      var range = sheet.getRange(3,1).insertCells(SpreadsheetApp.Dimension.ROWS);
      sheet.getRange(3,2).insertCells(SpreadsheetApp.Dimension.ROWS);
  }
}

What happens here:

• onEdit function runs when any cell in your spreadsheet is edited
• it receives e object as an argument, this object has a reference of which cell was edited (e.range)
• you can check in which column/row the edit was made by using e.range.getColumn()/e.range.getRow() in an if condition

More to read:

• onEdit trigger: https://developers.google.com/apps-script/guides/triggers#onedite
• event objects (e in our case): https://developers.google.com/apps-script/guides/triggers/events
• Range class: https://developers.google.com/apps-script/reference/spreadsheet/range