Currently, pt_PT uses wide day names also for abbreviated forms, both standalone and formatting.
For example, "sun" (Sunday, abbreviated-formatting)
It's "dom" for pt, "Domingo" for pt_PT. For comparison, month names have abbreviated forms
This was original reported as a bug in the datetime picker component in Firefox
Either delete or use the Mozilla names: "Seg", "Ter", "Qua", "Qui", "Sex", Sáb", "Dom".
Per our linguist:
The choice to use “segunda”, “terça”, “quarta”, etc., as the Abbreviated forms was a deliberate one as these are very common in software and “segunda” is much more recognisable than “seg”. Also note that these 3 letter abbreviations are already used in the Short format.
Being a native European Portuguese speaker I say your linguist is wrong, and it's obvious that on a calendar "segunda" isn't in any way more recognizable.
The short format does not use 3 letter abbreviations
So that is not a solution.