Is there any single word in English for flashbacks within flashbacks?
I've heard the term "nested flashback" before (as in "nested stories"). A non-authoritative example:
That's not a single word, though...
According to tvtropes.org, a flashback within a flashback is called an "Inception Flashback."
In computer programming that would be called
Recursion of flashbacks.
Or more descriptively, having a flashback within a flashback within a flashback would be
Third level of flashback recursion.
Perhaps, in a less technical situation, the lyrics of the song "The windmills your mind" could offer some guidance.