Device is used to refer to a piece of equipment or a tool that does a particular job, and it may operate by electrical power or be used by hand. Gadget is used to refer to a small piece of equipment that does something useful and it’s cleverly designed. Machine is used to refer to a piece of equipment that does a particular job, and it operates by electrical power. It is usually big and stays in the same place.
device The police uses a device for detecting blood alcohol levels in drivers. A mobile phone is a useful personal electronic device. The living room has a temperature control device.
gadget The iPad is a cool gadget. This is a neat gadget for opening cans without much effort. I bought a new gadget for sharpening knives.
machine This washing machine is what we need right now. I left a message on your answering machine. You can buy drinks from this vending machine.