No. Single threaded performance is the most important factor and the 5000 series is significantly faster than the 3000. Thus the 5600x yields better performance in Flight Simulator and is significantly cheaper than the 3900x. Flight Simulator simply doesn’t use those extra cores, so if you want to save a buck get the 5600x or 5800x rather than a processor with more cores.
The 5000 series from AMD currently has the best single threaded performance, making it the best processor you can currently get for Flight Simulator. It’ll likely be overtaken by Intel shortly, but for the moment, it’s tops. The 5600x at $300 with a $150-$200 AMD chipset is probably your best bang for the buck option, in addition to being currently the fastest setup. You’ll then be GPU bound with that 3080.
Price per core, I heard it’s the “worst deal”. But it’s between the cost of the 5600x and 5900x so whatevs.