Okay, so imagine you have three older family members, like maybe your grandpa, grandma, and uncle. Each of them has something important they want to tell you before they die, and they write it down in a special letter just for you. Those letters are called testaments.
Now, in some religions, there are three important guys who are like the grandpas of the whole religion. They're called patriarchs, which is just a fancy word for "important old guys." Their names are Abraham, Isaac, and Jacob.
Just like your family members writing you letters, these patriarchs wrote down special letters too. But they didn't write them just for you, they wrote them for the whole religion to follow. These letters are called the Testaments of the Three Patriarchs.
These letters talk about important things like how to be a good person, how to worship God, and how to treat other people. They're like a guidebook for how to live a good life according to these three important guys.
People in the religion still read these letters today to learn about what the patriarchs thought was important and how they lived their lives. So basically, it's like getting advice from some really wise grandpas who lived a long time ago.