If you have to play online, do not play ranked. The vast majority are just people looking to get those points and trying to gimmick and guess their way in a first to one. You will be learning extremely little at a very slow pace. Play endless and look for good players. Ask them what you did wrong, what you could be doing instead. How to deal with certain mix ups. Stuff like that.
Watch replays of your matches when possible if you’re not just getting off of SF. It really helps identify bad habits and tells you what you need to work on. Try to avoid watching replays when you’re in the middle of playing, as your judgement may be clouded.
And if you find yourself losing to someone who throws you over and over. Or DPs through a supposedly true block string every time. These things online are basically impossible to beat sometimes. It’s really a guessing game and most of the time, you have to just try laming it out if you run into players like these. Don’t factor them into what you could be doing differently unless it’s happening in 8 out of 10 games you played (but in ranked, 8 out of 10 players are basically just using online tactics anyway, so this is another reason to avoid it.)
tl;dr = be careful of what you take from online.
Also, stick to one character for now. Picking up a secondary is okay after you get familiar with that character, but three characters is kind of pushing it.