All three words are often used to describe something that causes death or is likely to cause death. However, there are subtle differences in their meanings: - "Deadly" often refers to something that is capable of causing death, but not necessarily certain to do so. For example, a "deadly disease" is one that has the potential to cause death, but not every case of the disease results in death. - "Mortal" is typically used to describe something that is certain to cause death. For instance, a "mortal wound" is one that will inevitably result in death. - "Fatal" is similar to "mortal", (you could also say a "fatal wound" which means the same as above) but it can also be used more broadly to refer to something that has major or disastrous consequences, whether or not those consequences involve physical death. For example, a fatal error in a computer program may cause the program to crash, but it doesn't necessarily result in actual death. Overall, these words are very similar and more practice will help you understand the nuances.

"Mortal danger" is a common collocation, but not the others.