Dear Sir or Madam,
I feel so sorry to tell you that I would like to know if I could ask for a casual leave of absence for three hours,for some family matters happened urgently and I have to go back home to solve them.As for the work I may miss,I will make up for it later on,please set your heart at rest.I will appreciate for your understanding.
Yours sincerely
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供楼主参考，不知道字数够不够。