Plagiarism or self-plagiarism, when there are multiple non-coinciding authors
If A, B, and C write paper X together, then they are all taking responsibility as co-authors for its content, which effectively belongs to all of them jointly.

Now, let's say that A reuses text in an inappropriate way in paper Y, authored by A, D, and E.  Even if A wasn't the one who originally wrote the reused text, A was certainly part of the editing and revision process and has actively affirmed that this text is a good expression of A's understanding of the material in paper X.

I would thus count such reuse as self-plagiarism: the material from paper X is part of A's prior work, and the issue is that this text has been reused in an uncredited manner.

In fact, I would even count it as self-plagiarism if is was D or E that copied the material (this situation could actually happen fairly easily, if D or E copies the material as "placeholder material" into a first draft, expecting it to later be revised).  Again, the supposition is that A, as an author of paper X, will know their own work and has a responsibility to ensure that it is properly credited or properly paraphrased in its new context in paper Y.