An actual literal triangular cross-section would be unpleasant to hold because of the sharp edges, I think. So you need to get rid of those somehow. If planar s

An actual literal triangular cross-section would be unpleasant to hold because of the sharp edges, I think. So you need to get rid of those somehow. If planar surfaces are the easiest things to make, then the best you can do is to cut off the corners with straight lines, at which point you have a hexagon. Not necessarily a regular hexagon, though. You might have long sides alternating with short ones. But a regular-hexagon writing tool behaves sufficiently like a rounded-off-triangular one for many purposes.