What are 3 things wrong with Descartes proof about the triangle concept? Philosophy homework help

I need a person that is familiar with Descartes, especially with one of his proof about God’s existence. The one about when he leads up to the proof with this analogy: just as the idea of “triangle” contains within it the idea “the sum of interior angles = 180” …so the idea of a “perfect being” contains within it the idea “must exist.” 

So I need three specific things that is wrong with this proof (has examples for each 3 things as well) This means you disagree with this proof since it is not convincing, there has to be flaws. I can only think of one good one, but I need more. Elaborate on each!