Disagree with most of what has been said above. I think it's totally your decision. It terms of employment prospects, there's no difference between the three of them - trust me. I was at QM, girlfriend was at KCL - I have a good idea especially of these two universities. Having actually gone through the application process for law firms and other organisations, and having actually gotten a training contract, I have a good idea what they look for and they don't value the university you go to particularly as long as it's good and you have a minimum of AAB/AAA at A-level.
So it's totally your decision - there won't be any difference in quality between the law courses at the three universities you mentioned. However, to make a few suggestions - if you want to study in London, go for KCL or QM, if not, go for Bristol. London is huge, full of all different people, but Bristol does a have reputation as a student city, and you could choose to still work there later on, so you do get more of a change of scenery. If you decide on London, QM probably will be cheaper if money is a bigger concern, and it's a campus uni - so you get the best of both worlds being in London as well as everything being very convenient and close. The area is better than it seems at first, there are lots of supermarkets and good restaurants nearby but central is on your doorstep. On the other hand, KCL is in a more prestigious and aesthetic area - the Royal Courts of Justice are a 2 minute walk from the uni. The Thames is next to the uni; the Waterfront bar overlooks it, and Tutus is a pretty decent student club above it. The law school is moving next door (away from its quite frankly not particularly previously aesthetic buildings) to Somerset House, so you'll be housed in a really beautiful building for three years too. However, students usually do have to commute to uni more and tend to live either in East or South London for their second and third years.