The first step is to determine whether your idea is actually "patented" as you assume. Make sure that the patent you are referring to is an issued patent (as opposed to a published application) and that if it is issued, that it's still enforceable. Also, the scope of patent protection is determined by the claims of an issued patent, not what is in the drawings and/or specification, so you need to look to the claims to determine whether your idea might actually be covered by this patent. I would suggest you talk with a patent attorney before making any determinations regarding patentability and/or potential infringement of your idea.
(For more info on determining whether an idea is patentable check out my blog post: Are Ideas Patentable? Is Your Idea Patentable?)
As to your second question, patents may relate to a given technology (e.g., 3D printing), but the patent coverage is typically more narrow that it initially seems based on the specification and drawings (again, the patent claims define the true scope of protection). In short, all these 3D printer patents that you refer to protect their own discrete aspects of of a given technology.