That's a bit of a strawman since I don't compare the three. I only compare Firefox and Brave. Of course Google is collecting the most data out of those and the tracking is worse, especially since they have other data points to compare it with.
However, according to my logic using Brave + Firefox simply must cause more data to be collected than using only one of the two, no?
I use different browsers for different websites / web applications.
For bank or accounting, for example I use Firefox in a container.
I configured Firefox to clear everything on closing : cache, cookies, history.
I don't save the passwords in the browser. I keep them in Vaultwarden, installed locally on a miniserver.
I'm currently trying out LibreWolf. It is based on Firefox, but with extra privacy and security features. I also installed Vivaldi, also if I am not a big fan of all these extra features integrated : mail, calendar and notes.
However, according to my logic using Brave + Firefox simply must cause more data to be collected than using only one of the two, no?