Using these three points (treatment of the crew, impressment, threat of mutiny), argue the effect of British maritime law on the death of Billy Budd.

Impressment is a critical theme of Billy Budd, as well as the brutal pragmatism that lies behind British maritime law. It's worth noting that Billy himself is not a volunteer; he's impressed into the British navy. Additionally, consider the ways in which Melville himself illustrates the brutality of naval life (I'd point to a particular incident found in chapter 9, just to give a potential starting point), and add that there's a strict hierarchy which defines naval life, by which the officers and common crewmen are set apart from one another, with an uneasy tension present between the two groups in question.
The third point of your question regards the idea of mutiny, and here I would begin by taking note of the degree of detail which Melville puts into establishing his setting. Billy Budd takes place in 1797, with Great Britain at war with Revolutionary France, in the wake of the naval mutinies at Spithead and Nore. Furthermore, I'd add that, given careful reading of the text, you should hopefully observe that insurrection emerges as a recurring theme in the story.
So, these three subjects tend to be interwoven across Billy Budd, and together they establish a picture of life in the British navy. It is brutal, it is harsh, but that brutality is itself based in pragmatism. The key scene, here, in which the entire story ultimately comes together, lies in the drumhead trial, where Billy Budd is sentenced to die.
Here I would begin by noting the conflict that emerges between regular law and martial law. All involved are actually in agreement that Billy does not deserve to die, and yet the punishment is death all the same. (In fact, the argument Vere makes during the trial scene is, essentially, that Billy has to die, because the laws of the British navy demand that he dies. To him, this is inescapable.) I'd suggest the key to answering this question is to read that drumhead scene very carefully and read into the arguments which Vere makes, where we see these undercurrents of brutal pragmatism on display. The harshness of British recruitment methods, the fear of mutiny, and the harsh brutality of maritime law, as they relate to times of war all come together in this scene, where we see the rationale which has shaped the brutality and how this entire picture fits together.