I'm having a blonde moment

[Galium]

Also possible that they are a child of a third cousin - so they would have the same common ancestor/s as your 3Cs, but a generation down from you.

[Gadget]

Quite -  generational differences should always be considered, as should relationships from more than one line.

Gadget

[Petros]

You get substantial variation even in one line.

On Ancestry i have 31 confirmed descendants from 1 set of great grandparents.
The 9 3rd cousins range between 22 and 125 cM,
the 3C1R between 13 and 92 cM
yet the only 2C1R on this line is a 57 cM match