Great question!

I believe that Hootsuite might be one of your best solutions. It is a free tool and you can publish through several social media platforms at once. For a free account, however, they limit you to 3 accounts I believe: https://hootsuite.com/plans/free

There is also Buffer as well which I believe has similar functionality: https://buffer.com

Here are some additional helpful resources for you:
https://blog.capterra.com/top-free-social-media-management-software/
https://www.socialmediaexaminer.com/5-social-media-management-tools-to-save-time/