So, you are considering engineering, computer science, and medicine, all at the same time? I think your bigger problem is choosing a career path, not choosing your A-levels. Do not make the mistake of attempting to keep every door open - trying to keep on top of the work for all three courses will inevitably lead to you falling behind on all three. You do not want to be a jack of all, king of none. If you were considering light subjects like the humanities my advice would be different, but the reality is that if you want to get into one of the stated (highly competitive) Oxford courses you must pick the correct A-level combination, and trying to keep options open will weaken the combination.
That being said, your current selection will only be competitive for medicine. It will not be competitive for either engineering or computer science. For computer science, you practically must have Maths and Further Maths A-level, and ideally computer science. For engineering you must have maths and physics A-level.
To reiterate: decide what course you want to do first and then pick your A-levels. Do not try to keep lots of options open indefinitely if you have an ambition to go to Oxford.