I found 6 that meet the requirements stated in the OP.
Buy ASRock B450M Steel Legend AM4 AMD Promontory B450 SATA 6Gb/s Micro ATX AMD Motherboard with fast shipping and top-rated customer service. Once you know, you Newegg!
www.newegg.com
Buy Used - Like New: MSI PERFORMANCE GAMING B450I GAMING PLUS AC AM4 AMD B450 SATA 6Gb/s Mini ITX AMD Motherboard with fast shipping and top-rated customer service. Once you know, you Newegg!
www.newegg.com
Buy ASUS ROG STRIX B450-I GAMING AM4 AMD B450 SATA 6Gb/s USB 3.1 HDMI Mini ITX AMD Motherboard with fast shipping and top-rated customer service. Once you know, you Newegg!
www.newegg.com
Buy GIGABYTE B450 I AORUS PRO WIFI AM4 AMD B450 SATA 6Gb/s USB 3.1 HDMI Mini ITX AMD Motherboard with fast shipping and top-rated customer service. Once you know, you Newegg!
www.newegg.com
Buy ASUS TUF X470-Plus Gaming AM4 AMD X470 SATA 6Gb/s USB 3.1 HDMI ATX AMD Motherboard with fast shipping and top-rated customer service. Once you know, you Newegg!
www.newegg.com
Buy ASRock X470 Taichi Ultimate AM4 AMD Ryzen 3000 Series CPU Ready ATX AMD Motherboard with fast shipping and top-rated customer service. Once you know, you Newegg!
www.newegg.com
Every one of these will work well for the OP's needs.
not true. the gigabyte board is not good enough. i have it (see attachment). it does not feed the CPU enough power. the 3900X
should score around 7100 in cinebench R20. on the gigabyte B450-i it gets between 6300 and 6800. always a random number. i thought my board was bad or the CPU was a dud. the CPU checks out fine in a low end lenovo desktop board with the same SSD* and RAM. the replacement gigabyte B450-i i got is equally worthless. does the same thing.
i also tried other benches and tried running an adobe premiere job. the gigabyte board comes up short every time. i started this thread
specifically because the gigabyte B450-i that i have does not do what i need it to do.
those asrock boards are probably even worse. i've seen reviews of those boards, from both professionals and users, saying they gimp R7 CPUs. R9 need not apply. they both use the same worthless 3 phase VRM, with doublers, and terrible cooling.
back to the topic, thank you everyone for the input. there is some amazing in-depth information here. i learned more than i thought i would. i will go with the MSI board. i will only need 1 M.2 slot and don't care about the audio chipset. i will update this post to let you guys know how it handles the 3900X.