Thanks @Lillie_Harris and @MarcLarcher.
I should have searched further, I was looking for “triplets”, but yes, the answer is in the more generic “tuplets” of Engraving Options. I was able to do exactly what I wanted, because of the various options for brackets and hiding.
I bow to the notation conventions, of course. It’s just that, in this particular case, those triplets occur all the way through, in every bar. At first sight it looked unnecessarily cluttered. So I took advantage of Dorico’s options to hide them after the first few instances, and I’m happy with that.
So I guess that in this case, I nod to the conventions rather than do a complete bow. I’m very happy that Dorico gives me that option.
Great software, fabulous community, thanks again!