If you look at a 3R enclosure you will find drain holes in the bottom. As far as drains go, there are a number of them available for classified areas and a few for non-classified areas. Stahl makes a plastic drain for threaded conduit systems, but I have not been able to find it at the supply houses. It comes with the power connect kits for some brands of heat trace.
This doesn't meet code, but I have used conduit plugs with small holes drilled in them for drains and also have cut a small notch in the bottom part of a LB gasket to make a drain. Our conduit couplings are straight thread and not watertight. Any time you have a coupling in threaded conduit on a vertical run in a wet location, water will enter the raceway.