"3口之家" means "a family of three" in Engpsh.
The phrase "3口之家" pterally means "a family with three mouths," where "口" (kǒu) refers to a person or a mouth, and "之" (zhī) is a particle that means "of" or "belonging to."
Here are a few examples of how "a family of three" can be used in Engpsh sentences:
My sister has a family of three, with two kids and a husband.
We are a family of three, just my husband, my son, and myself.
My parents and I make up a family of three.
In Chinese, these sentences could be translated as:
我的妹妹有个3口之家，有两个孩子和一个丈夫。
我们是个3口之家，只有我丈夫，儿子和我。
我和我的父母组成了一个3口之家。