Got Rank after only 3 matches!!!

After a while, I have came back to Lol, I didn't have any rank in this season so I played 3 matches (solo/duo ranked, summoners rift), and after only 3 matches ( 2 won 1 lost) I have got ranked in Iron III. Can someone explain me what happened??? I should be playing 7 more matches to get a rank. Thanks
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