"我家有三口人" 的英语表达是 "There are three people in my family."
这个短语是由两个部分组成的：
"There are"，这是英语中表示存在的常用词组，可以译为 "有"。
"three people"，这是表示数量的词组，可以译为 "三个人"。
例句：
"There are three people in my family." (我家有三口人)
"There are five cats in the house." (房子里有五只猫)
"There are two cars in the garage." (车库里有两辆车)