[mochimochi]

Are these sentences correct and do they mean the same?

She got married before she was twenty.
She had not been twenty before she got married.
She was not twenty before she got married.

I think the first sentence is right, but the others are not.
Am I right?

I teach a sentece such as "We had not gone far before it started to rain".
So some student use this sentence structure " had not pp before".

I don't think this structure can be used in every situation.
but I can't explain why.

Will you teach me why?

Thanks in advance