Hi Tienvn, Welcome to English Forums. I hope you'll find lots of answers to your questions here. Adjective order has some room for variation, but in this case 'first three' is much the commoner order (try googling them both), and the one whose logic is better, since you wish to refer to the first (set of) three paragraphs, whereas it is hard to imagine three paragraphs, each one of which is first.
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