wolf- I thinkm that a girl is converted under three (eiunah reu-ah lebiah)it may be different
Rambam (Hil. Issurei Biah 18:1) seems to make it a categorical rule that any non-Jewish woman is a zonah. He later says (7) that in order for a woman to become a zonah through forbidden sexual activity, she must be three. But this is a case where she is not a zonah because of forbidden sexual activity, but because of a definition.
Nonetheless, I could be wrong. If you have something different, I’d love to hear it.
The Wolf