@schratze Only in euclidean flat space. Simplest example of a triangle that does not satisfy this is one that you draw on a sphere with one corner on a pole and two others a quarter of the equator away from each other on the equator. This one has 3 right angles, for a total of 270 degrees.

Such triangles have lots of fun properties wherever they pop up, if you like strange but relatively simple math I recommend reading up on Lobachevsky spaces.
@efi @AgathaSorceress