There are multiple questions with even more answers on this exact question at Stack Overflow. The simplest answer, which I failed to find again, looks like this. Add the following lines to your repo’s .git/config:
[code lang=”bash”]
[alias]
pushall = !git push origin master && git push github master && git push remote3 master
[/code]
Simply change remote and branch names to fit yours and from thereon, a git pushall will do all the work.