Trichotomy is a type of thinking that suggests that when trying to understand something, there are three perspectives to look at it from. In Philosophy there are often three kinds of ideas that can be used to look at things. These three perspectives are called theism, atheism, and agnosticism.
Theism is the belief that there is a god or like a higher power that exists. Atheism is the belief that there is no god or higher power. Agnosticism is the belief that it is impossible to know whether or not a god or higher power exists.
So, trichotomy in philosophy means looking at an idea or way of thinking from three different perspectives or angles - theism, atheism, or agnosticism - to try and better understand it.