What Is The Difference Between Ground Beef, Ground Chuck, And Ground Sirloin?

Each cut oh meat comes from a different part of the cow. Usually from the sides to the rump and shoulders. Each section also has a different texture and flavor to it, so depending on the tenderness of the meat, will depend on the resulting flavor and texture.
While ground beef has added fats for leanness, chuck and sirloin don't, chuck is a marbled meat, (meaning the fat is marbled throughout the meat), whereas the sirloin is a leaner cut, and they sometimes add fat to this meat too, from 7%, to 25% in some cases.
I like the chuck because the marbling is already in the meat, and it seems to be a little more tender and flavorful than the beef or the sirloin. Hope this helps.

I am a vegetarian but I asked my husband who is a chef and he says:  Sirloin is the flank of the cow (sides), chuck is the rump (buttocks) of the cow and beef could be anything...leftovers.