I never use key signatures, only accidentals where/whenever they occur
I seem to remember having an “Atonal” key signature in Dorico 1, now trying out Dorico 3 I see no such thing. Where and how can I achieve this way of notating accidentals?

By default a flow in Dorico will be in open key and will therefore show accidentals where needed automatically. Options for how Dorico will treat accidentals are found on the Accidentals page of Notation Options.

For these kinds of relatively simple questions you should be able to get the answers you need more quickly by simply Googling for them (e.g. try Googling for “atonal open key Dorico”) or by searching in the online help here. We’re here for any complicated stuff or if you can’t find what you’re looking for after a quick search.