Just as the Horned God is considered an amalgamation of so many male horned deities from many different pagan religions, so is the Triple Goddess so many female pagan deities rolled encompassed in one being. The Goddess is considered triple because she is thought to represent three stages of womanhood.
I read on one page an author describe the phenomenon as something like the Christian Father, Son, and Holy Spirit all being distinct, but all being One. Of course, Christianity and Wicca are not the same religion, perhaps that analogy can provide some sort of starting point from which those of us unfamiliar with Wicca may begin to understand just like those of us unfamiliar with piping wouldn’t jump right into a job at this Local Plumbing Company Hamilton ON.