What I do in this case is use interactive rebase.
At your HEAD, create your branches A, B, and C. Also create a “backup” branch (you could name it backup) in case things go wrong and you need your original HEAD back.
git branch feature-a
git branch feature-b
git branch feature-c
git branch backup-before-rebase
Then, create a branch at the commit you want them to start from, maybe at a convenient stable commit. Call it new_trunk or something.
git checkout HEAD~50 ## this will be the new tree-trunk
git branch new_trunk
Then, do interactive rebases and pick out the commits you want to keep in that branch. Used this way, it’s basically like cherry-picking in bulk.
git checkout feature-a
git rebase -i new_trunk ## -i is for "Interactive"
When you’re done, you should have 3 branches with separate histories starting from new_trunk and a backup branch reflecting the old HEAD if you still need it.