How is the source configured for the Drop down?

For Cascading Dropdowns also you have to manage the logic. What if User selects the 3rd drop down first? Instead of first drop down. 

One way is write the logic on Change populate the source of drop down values by excluding the value selected in other filters.

Hi @TwinkleChawla

List of Values selected should be in parameter  .i.e , each time a value is selected , append the selected value to this parameter.

Ex: Param.SelectedValuesList="Value1","Value2"

If the dropdown is sourced from a  DPage/Report Definition,In the Report Definition, the below filter should be configured.

PropertyName "Not Equals" Param.SelectedValuesList

@TwinkleChawla

As per the question I understood, we need to write a logic to update the drop-down list every time. Configure the source on the drop-down as a clipboard page (PageList Property) and give the activity/report with an editable data-page name which gets executed when the drop-down is selected. Maintain the source activity to run only for the first drop-down and also configure the on-change action on each of the drop-downs which would execute a data transform /activity to filter the page list that doesn't have the selected property.

From the second drop-down, just mention the pagelist and do not give the activity in the source and make sure on-change action should be there.

@TwinkleChawla

There is a simple way to achieve this without writing any custom logic, by splitting the country selection and the country Actions/Comments into separate screens. However this might not be applicable for your use case.

1. In the first screen you can use a Multi-Select control to build a pagelist of selected countries. The control will automatically prevent you from adding duplicates.

2. After choosing countries via the multi select and submitting to the next screen (or configuring a refresh to refresh the table), the table will show your list of countries and allow updating the Comments/Actions

If you can't do that then you need to follow the advice in the other comments for defining custom logic to show only the un-selected countries, but the code to handle it will be fairly difficult to write.