You can also use the multi-account container addon (as mentioned earlier), a speed dial like Group Speed Dial, and a bookmarklet to contain a bookmark:
Last time I checked, it was possible to bookmark a URL with the name of a container, making Firefox open that page in the specified container from this bookmark. I can't find anything referencing t...
github.com
That is,
Create three containers, one for each Gmail account.
Go to the first container, log in to Gmail using the first account, and then run the bookmarklet to creat a bookmark. Copy the URL and create an entry in the speed dial. Do the same for the other two accounts.
You can also just bookmark the URL if you don't want to use a speed dial.
Finally, you can do the same for Proton and other platforms where you want to use more than one account.