Conditional Data Validation google sheets

From the sample I can see that it's just a copy paste of the code. The way it was designed was specifically to work that way, so you need to adjust the code for your purpose.

You might want to rewrite the code to be more accurate for your needs.

In the sample the onEdit() gets the column you are editing. Just as a side note, that can also be done easier. Instead of

function onEdit (){  
  var aCell = SpreadsheetApp.getActiveSheet().getActiveCell();

it could simply be (read up here)

function onEdit (event){  
  var aCell = event.range;

Then we look at the next important line that is the same for both options

var range = SpreadsheetApp.getActiveSheet().getRange(aCell.getRow(), aColumn + 1);

what this does is simply gets the next column range and that is where the rule will be applied. Now if you want the rule to apply to columns B C and D, all you need to do is adjust the getRange() like this:

var range = SpreadsheetApp.getActiveSheet().getRange(aCell.getRow(), aColumn + 1, 1, 3);

This will apply the rule to the all 3 columns. In .getRange(aCell.getRow(), aColumn + 1); the aCell.getRow() is for the current row, aColumn + 1 gets the next column after the one that was edited, 1 means get only 1 row and the next 3 says that get 3 columns. This however will only apply the exact same rule to everything. If you want to have different data sets for all 3 columns, you will have to adjust the other function:

function depDrop_(range, sourceRange){
  var rule = SpreadsheetApp.newDataValidation().requireValueInRange(sourceRange, true).build();
  range.setDataValidation(rule);
}

Currently it just sets 1 rule. What you will need is to create a rule for each range. You can either send it more variables or you can extract each column from that range and set a different rule on each.

Finally here is a sample of how I did it (basically the same as the example, just a bit more complex as it's something that actually is used in production)

function validateDynamic(event) {
  var spSheet = SpreadsheetApp.getActiveSpreadsheet();
  var shData = spSheet.getActiveSheet();
  var colTYPE = 2;          // column index for the Type entry
  var colMANUF = 6;         // column index for the Manufacturer entry
  var colMODEL = 7;         // column index for the Model entry

  var range;
  var newVal;
  if (event) {
    range = event.range;
    newVal = event.value;
  }
  else {
    range = SpreadsheetApp.getActiveRange();
    newVal = range.getValue();
  }

  var row = range.getRow();
  var col = range.getColumn();
  var validRange;
  var targetRange;
  var ruleName;

  switch(col) {
    case colTYPE:
//      shData.getRange(row, colMANUF, 1, 2).clearDataValidations().clearContent()
      targetRange = shData.getRange(row, colMANUF);
      ruleName = genRangeName('Type',newVal);
      if (newVal === 'N/A') {
        shData.getRange(row, colTYPE + 1, 1, shData.getMaxColumns() - 2).clearContent();
        shData.getRange(row, colMANUF, 1, 2).setValues([['N/A','N/A']]);
        setRule(genRangeName('Type',newVal),shData.getRange(row, colMODEL));
      }
      break;
    case colMANUF:
      var typeName = shData.getRange(row, colTYPE).getValue();
//      shData.getRange(row, colMODEL, 1, 1).clearDataValidations().clearContent();
      targetRange = shData.getRange(row, colMODEL);
      ruleName = genRangeName(typeName,newVal);
      break;
    default:
      throw('Dynamic Validation should not have run');
  }

  setRule(ruleName, targetRange);

  return 0;
}

And here is a screenshot of the data validation page (where I keep the lists for what should be in the dropdowns). This is way more conditional than what you need, but it's a different perspective :)

I build the named ranges with functions as well:

function genRangeName(str1, str2) {
  var name;
  if (str1 == '' || str1 == null || str1 == undefined) {
    name = str2;
  }
  else if (str2 == '' || str2 == null || str2 == undefined) {
    name = str1;
  }
  else
    name = str1 + '_' + str2;
  name = name.replace(/ /g,'');
  name = name.replace(/\//g,'');
  return name.toLowerCase();
}

and this is the function that actually sets the rule:

function setRule(ruleName, targetRange) {
  var validRange = SpreadsheetApp.getActiveSpreadsheet().getRangeByName(ruleName);
  var rule = SpreadsheetApp.newDataValidation();
  rule.setAllowInvalid(false);
  rule.requireValueInRange(validRange);
  rule.build();
  targetRange.setDataValidation(rule);
  return 0;
}

NOTE:

The above is the general explanation. I have walked the asker through this via chat on his spreadsheet. Here is the code of how the task can be accomplished. To anyone else finding this, keep in mind that this is very static. It only works if you have the data validation applied to columns B, C and D with the exact same names.

function onEdit(event) {

  var row = event.range.getRow();     //we get the current row of the range so we do not have to worry about it later
  var col = event.range.getColumn();  //we get the current column of the range so we do not have to worry about it later

//  In this section we simply check if the edited column was column A. If not we do not care for the rest of the code and stop the script with a return
  if (col !== 1) {
    return 1;
  }
//  ------------------------------------------------------------------------------------------------------------------------------------------------------

  var rule
  var targetRange
  var sp = SpreadsheetApp.getActiveSheet()

  if (sp.getIndex() !== 1) {
    return 1
  }

  if (event.value == 'Bend') {
    targetRange = sp.getRange(row, 2)
    setRule(targetRange, 'bend_dyes')

    targetRange = sp.getRange(row, 3)
    setRule(targetRange, 'type_of_bend')

    targetRange = sp.getRange(row, 4)
    setRule(targetRange, 'bend_size')
  }
  else {
    targetRange = sp.getRange(row, 2, 1, 3) //we want to clear all data validation if it was set to something else
    targetRange.clearDataValidations()
  }

}

function setRule(targetRange, ruleName){
  var ruleRange = SpreadsheetApp.getActiveSpreadsheet().getRangeByName(ruleName)
  var rule = SpreadsheetApp.newDataValidation().requireValueInRange(ruleRange, true)
  targetRange.setDataValidation(rule)

}