Neptunium is a radioactive element, and its compounds are mostly of scientific interest. Here are three common neptunium compounds:

1. Neptunium dioxide (NpO₂): This is the most stable oxide of neptunium and is a black solid. It is used in nuclear reactors as a fuel and in the production of other neptunium compounds.

2. Neptunium trifluoride (NpF₃): This is a green solid and is one of the most stable fluorides of neptunium. It is used in the production of other neptunium compounds.

3. Neptunium tetrachloride (NpCl₄): This is a yellow-brown solid and is one of the most common chlorides of neptunium. It is used in the study of neptunium chemistry.

It's important to note that these are just a few examples of the many neptunium compounds that have been synthesized and studied. Due to neptunium's radioactivity, handling and working with these compounds require specialized facilities and safety protocols.