They are all different cuts of beef. The "round" comes from the shoulder and "chuck" comes from the rump. Both of these muscle groups get worked a lot when the animal is moving around, and consequently they can be quite tough and but also very flavourful. Because it generally takes more time and effort to turn tougher cuts into good eats, these are usually fairly inexpensive as well. The sirloin is from the hip area, just forward of the chuck, and it doesn't get worked as much so it is more tender (and expensive) but somewhat less flavourful than the other two. Grinding the meat breaks down all the connective tissues in hard-working muscles and tenderizes it to a large degree. If you're going to make burgers or meatloaf or something along those lines, a good way to go is to blend a couple of different cuts to get a balance of tenderness and flavour. Try half sirloin and half chuck, see how you like it and then adjust future recipes as you like.