In a non euclidean / non flat universe unlike the one we appear to live in. Rules about triangles angles are affected by the curvature of space. In curved space the diameter could be curved to extend its length so it proportion to the circumference is exactly 3 times.
Please note I am not saying Pi becomes 3. As pointed out on other sites I have read, Pi is an irational constant whos value is set exactly to 3.14..... Its value as a constant is invariant within mathematics. For curved universes in which circle diameter to circumference ratios are different then a different constant is needed to distinguish - they are different universes with potentially different mathematical properties.