This should not be an issue. There are many lamp posts on the market which say “3-inch round” but in actuality the diameter may be slightly less (say 2-7/8 in.). This may be to account for expansion of metals during hot and cold temperature shifts. However, it is still safe to mount your Bentham Post Lantern on such a pole. Using self-tapping screws to secure the lantern should provide enough stability. Because rainwater cannot travel up the “gap” and into the pole, you should have no problem with water/moisture ingress.