Setup is simply 3 steps:
1. Sign up on each service, ideally with the same username.
2. For each repo you want to share, create the same repo name as a blank repo; do not automatically create a README.
3. Edit your local file .git/config to add push URLs, then push as usual.
Example:
[remote "origin"]
url = git@github.com:foo/bar.git
pushurl = git@codeberg.org:foo/bar.git
pushurl = git@github.com:foo/bar.git
pushurl = git@gitlab.com:foo/bar.git
fetch = +refs/heads/*:refs/remotes/origin/*