Hello @ricardof and welcome 
Please let us know your system information:
inxi --full --admin --filter --width 80 --color=0
For investigations, these outputs are important:
Xorg
export logs=( "/var/log/Xorg*.log" "$HOME/.local/share/xorg/Xorg*.log" )
for x in ${logs[@]}; do echo -e "\n---$x---\n$(cat $x)"; done
journalctl --boot 0 --grep="\([A-Z]{2}\)"
Driver
mhwd --list && mhwd --listinstalled
Kernel Messages
journalctl --grep="amdgpu"
journalctl --grep="nvidia"
Please share it also.
Just a guess, since you are new, but did you install nvidia driver, do you use the open source nouveau driver? Nvidia RTX 3070 is not supported by nouveau: FeatureMatrix · freedesktop.org and therefore understandable not working correctly.
You need to install the nvidia driver (which is optional):
Automatic detection:
sudo mhwd -a pci nonfree 0300
or choose the correct one:
sudo mhwd -i pci video-hybrid-amd-nvidia-prime
For switching the GPU, you need to install a tool:
pamac install optimus-manager
or work with xorg config files, like in that example:
Oh well and don’t forget this on hybrid GPUs:
Add/Replace MODULES=() with:
MODULES=(amdgpu nvidia nvidia_modeset nvidia_uvm nvidia_drm)
at /etc/mkinitcpio.conf (open it with an editor as root)
Then run sudo mkinitcpio -P to update the initramfs.
This could solve some timing issues with the nvidia driver, which could results i crashes.
Good Luck