Autogenerate complex Date / Count based Title
Unless I'm missing something, you should be able to use the seq Twig function (docs) and use something like:
{{ 'now'|date('Ymd') ~ '-' ~ seq('now'|date('Ymd'), 3) }}

This would generate the format you need YYYYMMDD-NNN and reset the NNN every new day.
Sequences are stored in your sequences database table.