この ＿＿＿ ＿＿＿ ＿★＿ ＿＿＿ ください。

Translation: Please buy three of these apples. - Sentence assembly: The correct order is 'この [① りんご] [② を] [★ ③ ３つ] [④ 買って] ください。' - The standard order for direct object and quantity before the action is: [Noun] + を + [Quantity] + [Verb]. So: 'この りんご を ３つ 買って ください'. - The third slot (★) is filled by '３つ' (Choice 3). - Therefore, the answer is '３つ'.