Interim reference letter?
I will leave my company at the end of April 2020 and I would like to ask for interim reference letters (instead of a final one with all the positions I covered) as during my staying with them I covered 3 different positions. Is this possible? Can they refuse to deliver them to me?
No - your management hasn't the right to refuse you an interim reference letter! Upon Art. 330a Abs. 1 CO (Obligationenrecht) you have the right at any time to request a certificate about the nature and duration of your employment.
Furthermore the interim refence letter has to be - as the final one:
complete (naming all ...
So if I understand you correctly, you want one (interim) reference letter for your current position plus retroactively an interim reference letter for each of your two previous positions?
You can ask for an interim reference letter at any time (as Marc F. already wrote) but the letter is then issued based on the current situation (or from the beginning of your employment until now).
Of course, you can always ask, but I think that your employer could refuse to issue the retroactive letters.
Now it's too late but in general, I would always recommend request...