I've been working on a project, but unfortunately, I forgot to switch to my branch, and as such have been working on master

How can I copy the work (3 files) I've done here from master, to my branch (called, for example branch123) without comitting to master?

Sounds like all you need is the following:

git stash
git checkout branch123
git stash apply

Then you should be back on your own branch without touching the master branch.