Getting a TOP3 using SMALL, based on multiple CRITERIA and ignoring DUPLICATES

[tisko]

Hi there,

I need a bit of help combining several functions to get a top 3.

I have a workbook with a lot of colums that keep track of progress on cases I work.

The relevant columns are simple:

Column A: Unique codes or ID's. I precreated 500 of them, so A1 through A500 are filled.
Column B: Startdate. Gets a manual value when I started something
Column C: Due date. Calculated, can only have a value if B has a value.
Column D: Finished date. Gets a manual value when I finish something.

I want to keep track of the 3 earliest due dates of cases that not yet have finished (which could possibly have expired). So the condition is an empty cell in column D.
I want the returned values to be the unique ID's of column A. Often due dates are identical, I want to prevent duplicates in the returned results.

I did some research and ran across a helpfull page on Ablebits, which explain the individual steps with INDEX, MATCH and COUNTIF and experimented with FILTER but I can't seem to get them combined into a working function.

Can anyone help?