+ All used to see inside a person without invasive surgery.
+ All can be used to diagnose issues inside the body and observe problems in the body.
+ All use a form of wave.
+ The waves all have different frequencies/wavelengths.
+ The image you get is different from each, because the frequencies are different so the body parts that the waves are absorbed by are different. This means xrays pick up bones better while ultrasound picks up body tissues better.
i dont know ct scans but for xray. they are only used on hard tissue such as bones and ultrasound is used on soft tissue such as muscles and organs. x rays use electrons fired at a metal plate but ultrasound uses sound waves which reflect off the tissue and then an image is created on a software