What do the following instruments detect ( i want to understand the differences among them): ultrasound, mri, & an x-ray?

Different physics : Ultrasound images come from reflected sound waves so show "echogenicity" of tissues. X-ray images are like shadows - x-rays are blocked by denser materials ("radio-opaque" like bone, metal) and give "shadows" of what is not as dense. Mri images reflect differences in the chemical composition of the tissue (brain's gray matter v white; heart v lung; fat v muscle) via magnetic fields ; radio waves.