To add to what Peter says if you are changing your house wiring you may need to pull a permit in your jurisdiction. Which may mean an inspection to ensure it's all up to code. I would suggest that you ask a local electrician about what you plan to do. Hopefully you know one.
Recently I had an idea for something and my electrician friend told me it would be a no go as the thing I would build wouldn't have been safety approved. Could be a big issue if I burn the place down and hurt someone.
Three way switches are wired differently than a two way. Typically requiring a three conductor wire between the switches at least. Sounds like you'd need to pull some new wire. If you don't know an electrician you may want to start making friends with one.
Electrical wiring is mostly very simple and straightforward but getting it wrong can be tragic. Like fixing your car's brakes, don't do it unless you know what you are doing.