While it is plausible for any of those agencies to spy on the domestic population, and it may definitely be possible that some of them need to be combined or removed, you must realize that any country will have such agencies as they have important roles, particularly for the military and law enforcement. It would be rather unwise if the FBI didn't have an intelligence branch because there's plenty of analysis to be gained from there, for example how drug cartels operate. Similarly, the DIA has an important job of accurately finding out the military capabilities of other countries. Whether they do bad/illegal things or not they are a necessary part of any government.