My best shot would be to say that all of them might be acceptable but I have to admit that this is one of the most difficult things in English for me...
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Nina_NiaI am going to be in Sydney one week before the beginning of the internship, I think 'it' ('this' 'that') will be enough to find a flat.'that' is typical when referring to something general already said — especially if there is no specific definite entity to refer to as "it".