Removing item from a list once chosen Dropdown Googlesheets Script - list

Question

I want to Remove an item from a list once chosen in the dropdown and then the next dropdown won't have that item anymore. I've already made something manually but it takes super long and is not really ideal because you would need to go down in order otherwise it gets all mixed up.
https://docs.google.com/spreadsheets/d/1o3J1ZXzxQL11pdN1FDOQZ7YXbSib6A8KwkxBzO5bSM0/edit?usp=sharing
I've made 3 tabs one where I explain what I want and the other were I made it manually so you have an idea on how it works. But remember the manually one you need to go down in order. I want for it to look at a range so it doesn't matter which dropdown I choose for it to get removed from the list.
If it isn't possible to do it by range then like the same way as manually but then in script form would be a great help as well.
Here is what I tried but I'm really new with scripting so I might not even be close:
var ss = SpreadsheetApp.getActiveSpreadsheet().getSheetByName('Dropdown');
//get's the value in range A1 ( the firstdropdown value )
var firstDrop = ss.getRange("A1").getValue();
//get's the value in range A3 ( the seconddropdown value )
var secondDrop = ss.getRange("A3").getValue();
//get the list ( I don't want to put the list in here sincec I'll be
//working with entries from google forms. so the list will be changing)
var list = ss.getRange("H1:H5").getValues();
if(firstDrop == list){
var newlist = (list-firstDrop);
}
}

Answer 1

This may work for you - I think it gives you the result you want, but its appearance isn't ideal. I've placed it into your sheet, in the last tab. I believe that this is the same approach as Mateo's earlier answer, but down with a formula instead of code.
=SORT(FILTER(B2:B;NOT(COUNTIF(A2:A;B2:B))))
where A is the column where you are entering data, and B has your list of valid names/words.
I use this formula to filter the list of words/names that show in each dropdown list. It starts with the original list, and excludes any that have already been selected. The data validation for your cells points to the filtered list, not the original list.
The drawback is that once you select a word/name, it is no longer eligible for the dropdown cells, so even the cell where it has already been applied shows it as an invalid entry, with a little red triangle. The benefit of a working solution may outweigh this though.

Answer 2

I believe it should look like this:
function availableSlots(){
var form = FormApp.openByUrl('URL_OF_YOUR_FORM'); // TODO add your form url
// need to read what slots are available
var slots = SpreadsheetApp
.getActiveSpreadsheet()
.getRange("slots!A2:C10")
.getValues(); // TODO make sure getRange matches your quota range
var choice = [];
// loop through our available slots
for (s in slots){
// test if slot still available
if (slots[s][0] != "" && slots[s][2] > 0){
choice.push(slots[s][0]); // if so we add to temp array
}
}
var formItems = form.getItems(FormApp.ItemType.LIST); // our form list items
// TODO assumption that first select list is the one you want to change
// change formItems[n] if you have more than one select list
// and we just rewrite all the options to ones that are free
formItems[0].asListItem().setChoiceValues(choice);
}

Answer 3

As I was not sure which dropdown menus you wanted to use I have created a sample sheet with your requirements (photo attached below). In order to reduce the list of items in function of the output of the rest of the dropdowns I have used on onEdit() trigger to check for any changes in these dropdowns and in function of these cell values remove the required items from the list.
The following piece of code has self explanatory comments that address how I have implemented this:
function onEdit(e) {
// Get sheet
var sheet = SpreadsheetApp.getActive().getSheetByName('Share your Ideas here :)');
// Get list from range. Flat will make the 2D array returned by getVaues() into a 1D array
var list = sheet.getRange("E1:E5").getValues().flat();
// Get values from the dropdowns
var a = sheet.getRange("A1");
var b = sheet.getRange("B1");
var c = sheet.getRange("C1");
// Check if any of the values in the dropdowns match the list and if so remove them from
// the list
var index = list.indexOf(a.getValue()); if(index != -1){list.splice(index,1);}
index = list.indexOf(b.getValue()); if(index != -1){list.splice(index,1);}
index = list.indexOf(c.getValue()); if(index != -1){list.splice(index,1);}
// Create the data validation rule with the filtered list
var rule = SpreadsheetApp.newDataValidation().requireValueInList(list).setAllowInvalid(false).build();
// Create/update the dropdowns with the right filtered list
a.setDataValidation(rule);
b.setDataValidation(rule);
c.setDataValidation(rule);
}
References
Data Validation
requireValueInList