How do I install minecraft 1.7.10 and forge on a raspberry pi 3. Since I have this friend who wants me to install 1 mod to play on his server.
You should be able to just install it regularly, i.e. download forge from the site, run the jarfile with java -jar (forge-installation).jar
but keep in mind, to use 1.7 forge requires java 7 iirc.
edit: apparently only some builds of forge work, and you don't get sound. I found this information from https://html.dynu.net/index.php/how-to-setup-minecraft-on-raspberry-pi-forge-version-1-12-2/, but it should apply to 1.7 versions as well. Just experiment a bit (and maybe follow the url for the guide i listed, but with a different download).
another edit: apparently it doesn't work that way... try following the guide but using the download for 1.7 forge