Stick with 3, universities only take 3 grades regardless and it's better to focus on 3 and get stronger grades.
For Russel Group universities, more academic subjects would help you, but probably won't be required (aside from English lit). I'd primarily suggest history, philosophy and a language, or even psychology or economics; as these subjects give you the opportunity to further your writing, critical analysis and world awareness skills- all of which you would need in journalism. Ultimately though, take whatever you enjoy and can achieve the highest grades in.