Well, if we know that an obtuse angle is an angle greater than 90 degrees, and that a triangle's angles MUST sum up to 180 degrees and contain three angles. Then it logically follows that there's no way for a triangle to have 3 angles AND have two of the three be greater than or equal to 90 degrees as the remaining angle would have to be zero degrees or some "negative angle"- which I am pretty sure doesn't exist lol! As for a proof, I think you can cook up something with what I have given you so far :)
