First, absolutely make sure your PSU is powerful enough - instant shutdowns like yours could indicate an issue with it. Maybe replace it. RTX 3090 can have spikes up to 500W and that means, along with your CPU, that your PSU must be rated at the very least 850W.
Speaking of your temps.
Your CPU is running close to its rated maximum, which is 90C, which means you'd better improve your case cooling by installing case fans e.g. 120mm (140mm are beter - quieter and more powerful) and probably installing a better cooler on your CPU along with changing thermal paste - my preferred one is Arctic MX-4 (MX-5 in theory provides better performance but it's a lot more cumbersome to apply).
Installing proper case cooling might prove enough since your GPU is definitely increasing your CPU temps.
Don't forget to update your EFI BIOS as well.
You can also use a software only solution: enter your BIOS and
- either decrease your CPU PPT (maximum wattage)
- or set the maximum temperature for it, e.g. 85C
Both will result in decreased multithreaded performance but not so much. You may get more help here: https://www.reddit.com/r/Amd/