How long have you been playing? How many hours have you put in?
I’m guessing the answer is not many. Fighting games are difficult and require dedication just like anything else. You can’t play golf for two weeks and expect to be plus handicap, you can’t lift for two weeks and expect to have an awesome bod. Most of the people who are beating you have been playing fighting games for 5+ years, or at least since the respective game came out. They’ve put in work. If you’re looking for quick results, fighting games are not the genre for you.
If you really do want to learn and put in the time, I suggest sticking to one game and putting all your effort into becoming good at it. Right now you’re playing three games which have basically nothing in common with each other; you’re spreading yourself too thin. Pick a game, go to the best resource for that game (SRK for UMvC3, Skullheart for SG, 8wayrun (I think) for SCV), learn everything about it. Practice in training mode every day, and watch videos of high-level players who play your character to see what they do differently from you. Play against real opponents and apply what you have learned, making a conscious effort to reform your bad habits.
Fighting games are a constant journey of self-improvement, and the journey is not for everyone. Hope you stick with it.