He invited 10 friends but he didn't need to cook for 1 of his friends so that left 10-1 = <<10-1=9>>9 friends to cook for.
He was cooking 3 burgers per 9 guests so 3*9 = <<3*9=27>>27 burgers
He needed 27 burger buns but 1 guest wouldn't eat the buns so now he needs 27-3 = <<27-3=24>>24
He will need to buy 24 buns and they come 8 to a pack so he needs 24/8 = <<24/8=3>>3 packs of buns
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