You are correct that ryzen 3000 doesn't have quad channel memory, however it does benefit from two ranks per channel.
So having 4 sticks of 8gb's of ram will yield 1 rank per stick, 2 ranks per channel (dual channel), at which point the system will leverage the page file on the ram stick in something called "Interleaving" giving much better performance at a given clock speed.
Tests have been made and quad rank kits at 3000mhz speed in a dual channel platform performed similarly to a dual rank kit running 3600mhz speed on the very same platform.
On Intel you can even go octa-rank (4 ranks per channel, so 2x16gb's per channel, 64gb's total) on the consumer platform and see performance equivalent to 3600mhz running 64gb's of 2666mhz ram.
I'm not sure if Ryzen 3000 takes advantage of octa-rank, it does take advantage of quad rank however.
So big gains can be made even without quad-channel. Makes you wonder the whole point of high speed kits of ram, they are harder to run, more harsh on the IMC and end up being beat by a similarly priced higher capacity and easier to run slower kits.
16gb's 3600mhhz = 32gb's 3000mhz = 64gb's 2666mhz. (If my memory serves the timings were CL16-16-16-32, CL16-18-18-38, CL15-15-15-30)
Gaming performance is positively impacted too, especially in smoothness (0.1 and 1% low's so the biggest gains), synthetic benchmarks had mixed results, despite real world testing always favoring the multi rank configs, latency especially was worse the more sticks you had, despite gaming performance being much better.