function onEdit(e) {
  var Column1 = 1
  var Column3 = 3;
  if (e.range.getColumn() == Column3) {
e.range.setValue(Column1 * Column3)
}
}

function onEdit(e) {
 var sheet = SpreadsheetApp.getActiveSheet();
 var dataRange = sheet.getRange("A:C");
 var data = dataRange.getValues();
   for (var i = 0; i < data.length; ++i) {
   var rowData = data[i];
   var Column1 = rowData[0];
   var Column3 = rowData[2];
   if (e.range.getColumn() == Column3) {
   e.range.setValue(Column1 * Column3)
   }
   }
   }

function onEdit(e) {
  var Column1 = 1
  var Column3 = 3;
  if (e.range.getColumn() == Column3) {
    e.range.setValue(e.value * e.range.offset(0, Column1 - Column3).getValue());
  }
}