You’re spot on. The laser cutting hole diameter should not be smaller than the material thickness. Therefore, use a minimum of 3 mm holes in a 3 mm plate. But it’s more of a guide than a hard limit. With 316L and a fiber laser, you can usually go a bit smaller, maybe 2mm on a well-calibrated machine with a skilled operator. I would avoid 1.5 mm though. It’s doable, but you will need a slow traverse speed and Nitrogen as your assist gas to avoid oxidation because of the speed. Piercing gets trickier at that size, as heat buildup can cause taper and/or slag. You’ll likely encounter burrs, requiring high-effort post-processing.