I have an issue with call merge on the Pixel 7 as well. I am on AT&T. I can only figure out how to merge two calls.
Thereafter, the "merge" button changes to a "manage" button and when you select it, you get a list of the two calls that have been merged. I don't see any way too add a 3rd caller.
I switched to GrapheneOS from iPhone about 4 or 5 weeks ago. (I bought a used Pixel 7 because I didn't want to contribute to Google.) I am so excited. It has been a bumpless transition. I am using it as my primary phone and most of my call time is for work. I am loving it! I have never used Android so I am still figuring it out and that could be my merge issue, but other then this I have really had no stumbling blocks.
Conferencing calls together is something I do a lot of at work so I am going to keep trying and/or try to figure out a work around. Any suggestions would be greatly appreciated.