Hey Danielnoahsmommy raised good point! If going to go to all this trouble... go all the way! Anyway...to answer your question
If you want to add a TOILET to the third floor you are required to have a 3" waste stack to do this... meaning you CANNOT use the plumbing pipes from the kitchen sink or the washing machine waste line.
There is an exception or two here.. but doubt they apply.
Most likely will need to cut into a vertical sewer pipe from the 1st or 2nd floor and extend it to the third floor. That means you will need to open walls.
If you decide to open walls... and you have the space.. AS DNM suggested... a 3/4 or full bath may be worth considering considering work involved in adding just toilet and sink...
All depends on space available, of course!
Let us know what you think... MARK