This means that sometime during the three days we go to the library. 在美国,一般日常生活中遇到的数字几种不同念法: 很长的数字编号,比如电话号,ssn,驾照号这些,按照位数读,比如13012345678,就念one three zero one two three four five six. We could say 'once in three days'.
Three | search | faqs, products, events, locations mobile phones. My english teacher was kind enough to let me choose a song for a listening. We need 'in' to connect 'once'.
We might go on day 1 or day 2 or day 3.