Funny, I did just exactly that at work yesterday. If your branches have linear histories, here's what to do:

1. Make sure all branches touch separate files. I would strongly recommend git-filter-repo over git-filter-branch. It's way simpler to use and orders of magnitudes faster.

2. Generate the list of commits in the correct order:

git rev-list --date-order --reverse branchA BranchB BranchC > revisions.txt

3. Go to any branch and: git rebase -i --root --force-rebase

Paste the contents of revisions.txt into the sequence editor and add "p " at the bigging of every line. Run the rebase and you are done.