I actually popped into my local Microcenter last week and saw they didn't have the quad core Ryzen 3rd gen parts either, didn't even have a spot for them on the shelves. So I asked the salesperson when they expect to get them in. He had to go check, and he came back and said from what he could find, it looked like AMD was focusing on providing the CPUs to OEM partners before providing them directly to the DIY channel. So it may still be a while until they actually become available.
It's interesting, it took a while for AMD to release the Ryzen 3rd gen quad core parts, and even once they're released, availability is scarce. I get the feeling that it's because the amount of chiplets manufactured that failed to qualify with enough functional cores to be even just a 3600 was relatively low, so they've had to let supply slowly build up. That, and they may still have stock of the 3200G and 3400G they want to sell first, given that the 3300X/3100 basically kill off the APUs for anyone building a PC with a dedicated graphics card.
Also could be that they're waiting to push their stock to stores until they see just how well 10th gen Core i3 parts do by comparison.