Can I use DDR4-3600 RAM at 3200 speed?

Yes, it’ll work fine. The RAM will just run at the slower speed your CPU supports. Grab the 3600 kit if it’s cheaper or on sale — you’re not losing anything. At default settings it’ll boot at 2133 or 2400, then you set XMP to a 3200 profile or manually set speed to 3200 with safe timings and 1.35V. The only risk is forcing 3600, but running it at 3200 is stable.