(1) and (2) aren't correct, and it isn't very clear to me what you are trying to say. " "it was mine" in (3) is fine. "your things" feels vague and doesn't fit the context terribly well.
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GPYIt's possible that you mean "Just because a girl talks to a guy day and night, it doesn't mean that she loves him as well.Yes, I meant what you said above. Could we also say 'when' in place of 'just because'? For example; When a girl talks to a guy day and night, it doesn't mean that
LaboriousYes, I meant what you said above. Could we also say 'when' in place of 'just because'? For example; When a girl talks to a guy day and night, it doesn't mean that she loves him as well."Just because..." sets the sentence off in the right direction and makes it clear to the reader that a contrast is coming. "When ..." doesn't achieve this so well.
LaboriousIt wasn't your fault, it was mine, indeed. I misunderstood you, your smile, your things. (Is it correct to say 'it was mine' ...Yes. The series you need to replace a combination of a possessive determiner and a noun is mine, his, hers, ours, yours, theirs.