The Ryzen 9 3900 is nothing but a Ryzen 9 3900X with lower clocks, resulting in a significant TDP drop from 105W down to 65W. It is still a 12-core/24-thread chip with 6MB of L2 and 64MB of L3 cache, and all other bells and whistles. It works at 3.1GHz base and 4.3GHz Boost clocks, which is nowhere near the 3.8GHz and 4.6GHz on the Ryzen 9 3900X.
The Ryzen 5 3500X is a bit of a different story as this is a rather interesting SKU. Compared to the Ryzen 5 3600, the Ryzen 5 3500X is a 6-core CPU but without SMT (simultaneous multithreading), so it is stuck with 6-threads and comes with just slightly lower clocks, ticking at 3.6GHz base and 4.1GHz Boost clock. It still has the same TDP as the Ryzen 5 3600, 65W. There is also a possibility, that AMD might have a Ryzen 5 3500 in the works as well.
While the Ryzen 9 3900 will be available globally from usual OEMs, the Ryzen 5 3500X will only be sold on the Chinese market, at least for now. While AMD did not give out any specific price details, the Ryzen 9 3900 should fit nicely between the $499-priced Ryzen 9 3900X and the $399-priced Ryzen 7 3800X, while the Ryzen 5 3500X should offer a cheaper alternative to the $199-priced Ryzen 5 3600.