Full Idea
In the ZFC cumulative hierarchy, Frege's candidates for numbers do not exist. For example, new three-element sets are formed at every stage, so there is no stage at which the set of all three-element sets could he formed.
Gist of Idea
In the ZFC hierarchy it is impossible to form Frege's set of all three-element sets
Source
Penelope Maddy (Naturalism in Mathematics [1997], I.2)
Book Reference
Maddy,Penelope: 'Naturalism in Mathematics' [OUP 2000], p.23
A Reaction
Ah. This is a very important fact indeed if you are trying to understand contemporary discussions in philosophy of mathematics.