Seriously, you’re not playing a team. Your team is basically 3 individual characters. They are also all relatively high execution characters for their more damaging combos.
Pick one of the 3 and add a complementary assist. I’d suggest putting doctor doom on your team and using the hidden missile assist. It is a great assist to give you some leeway into learning the game.
The main thing you need to learn is how to call assists and keep them safe. Basically, when you call an assist you want to be doing something to put the other person in blockstun so that you can get your assist out. From there what you need to learn is converting any hit into a combo. To begin you don’t need fancy combos and once you get used to the neutral game you can move onto some of the fancier stuff from there.
Also, to learn the game, I’d suggest trying a character like wolverine, taskmaster, hulk, or maybe nova if you want to learn an airborn character on your team. They all have really high priority normals and pretty straightforward gameplans that will allow you to adjust to the game. I’d honestly suggest taskmaster because he can do a little of everything to learn the game. Nova can be a lot trickier but has great normals and flight.
Lastly, learn to air throw and convert them into full throws. Air throws (especially air throw option selects) are vital in this game.