If there is a design of a coffee cup for example and I wish to modify it and print it, would I be breaking the law? This question has been asked several times by designers but there are issues that can be involved when you do 3D print. If I say I want to print a coffee cup for my own personal use and only print one copy, then it would be seen as educational and fall under Fair Use. However, if I were printing more than one copy and selling the coffee cup, then yes I would be breaking the law and I would be essentially stealing someone elses design.
When 3D printing be careful as to what your purpose is, because selling someones elses design is illegal and stealing for example Disneys design will lead you to a court case and chances are you will be sued.