Proton VPN does not officially support Raspberry Pi, but you can try to set up their CLI version. https://raspberrytips.com/protonvpn-on-raspberry-pi/ I tried doing this a while back but it hung on the login step. I reached out to Proton and their only response was that Raspberry Pi is not supported. So, best of luck and I hope you can get it to work!
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says I need a raspi 4, I'll try openvpn first I think
If you give the lightweight os DietPi a shot, Proton VPN works via their DietPi-VPN tool right off the bat. You just need the IKEv2 username/password combo from your Proton VPN settings. It will always connect to the same server you specify in the settings as well. I believe OpenVPN should work the same but can't confirm. I'd definitely recommend DietPi if you're looking to self-host any services - especially on a 3B. It's super lightweight and has a tonne of graphical config tools that makes setup a piece of cake.
I can't install any other OS on it right now so I'll try openvpn
I can't recommend the official (or community) cli. Best would be to use wireguard or openvpn. In a docker setup, try gluetun
WireGuard also definitely works!