1) A lot of Muslims get married without knowing the correct way to do Ghusl and some don’t do it at all cause they are not religious. Is their Nikah valid?

2) Is there a condition that one cannot be in the state of Janabat when the Nikah is being done?

3) If one’s nikah is done while they are in a state of Janabat knowingly or unknowingly, is their Nikah valid?

1) Validity of ghusl does not necessarily have any bearing on the correctness of a marriage contract.
2) No
3) Yes