So in answer to some comments, I may slightly encroach
It must be hard to read, I'm sure your a very lovely person, but I have been sat on, squished many many times on the met line and have had to endure a long uncomfortable tube ride as I can't stand up for long periods of time due to a medical condition
I hate it, absolutely hate it when my personal space is being violated - you could look like Brad Pitt and if you come and sit next to me and your body is overspilling onto mine then I am going to feel very uncomfortable, I would feel less uncomfortable if you were woman but I still wouldn't like it. You see the thing is, I don't know you, I don't know your lovely all I know is that your body is touching mine and that it is making me very uncomfortable
From what you have said your being given more room, as people are getting up, so whilst I appreciate you think people are being rude to you by getting up, what is actually happening is that you are making them uncomfortable by encroaching "slightly" into their personal space as you call it.
Those "three" seats are not big enough for someone who is a size 24 and two people of a size 8 quite frankly, if your joining that person in the middle feels like a hot dog in a bun, so either sit facing the aisle and maybe ask the person your sat next too, do you have enough room, be considerate of your fellow passengers.