Technically, "it's" is a contraction of "it is". However, preceding "been" "it's" is used colloquially to mean "it has". "It is three years since I last went to the cinema." No. "It has been three years since I last went to the cinema." Yes. So, "It's been three years ..." is the way to go, assuming your audience will accept "it's been" as "it ... More @Wikipedia
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