Hey there! 1 and 3 are very natural. 2 is odd (as ‘before’ needs to link to a time - it doesn’t necessarily link to the present). From personal experience, ‘we used to date’ is the most common. ‘We used to go out’ is a popular equivalent in the UK. 😊

Yes, these all have the same meaning. We dated before is the weakest - 'before' what? We understand you mean before now, but the other two sound more natural.