So, tonight's the last night I have with Marvel vs. Capcom 3, at least until I purchase the game. Being a cheap, three day rental, I was surprised that I was sold on the game within minutes of assuming control of my on-screen characters. The only problem is that I just cannot seem to win online; my record is somewhere around 0-20 in Ranked matches, and have won less than five games in Player matches.

• I've completed up to level 7 or 8 for most of the characters in Mission mode, without any trouble. 
• In Arcade mode, I can set the difficulty up a little from the default mode, and do quite well. 
• I find that I'm fairly decent with a few characters, favoring Tron, Amaterasu, X-23, Morrigan, and Zero.

Probably not the answer you're looking for but the best thing you can do right now is keep playing ranked and keep losing. You'll become a better player for it. There's no magic tips, mostly comes down to experience.
Trials are fine and all but what use is a combo if you can never get in to perform it? Human oppostion > Arcade, they're nothing a like, the com doesn't fall for mind games, mixups, all the important stuff basically.
 
Typically you're time in training mode should be spent practicing combos, working on your execution, practicing setups to get in and perform said combos, and just messing around, getting creative, try and come up with some crazy new mixups or whatever but I wouldn't worry too much about the last one yet.
 
What platform are you on? If you're playing on PS3 I can try and help you out.

@OP;
 
You haven't really said what your weaknesess are when fighting?.. What kinda teams do you match up?.. Can you do combo's yet?.. Do you use the X-Factor?.. Do you know what everything does.. why it does it.. and when to use it?.. all of this stuff is really important.
 
I know a quick tip for now tho.. if you looking for the match.. make sure in the rank bar you have it set for same. That way you don't get someone waaay beyond your skill lvl.
 
:)

The problem with Training Mode and Arcade Mode and Mission Mode is that what you're doing is practicing to attack and attack and attack and attack and with the way MvC3 is designed, pretty much anyone can attack with a fair level of competence. The real hurdle in a vs. game is knowing how to defend all the crazy shit going on and the only way to learn that is by playing other people. Best suggestion I have is find a buddy to play with, get horribly destroyed over and over again and any time you don't understand how or why you got destroyed, ask your friend what they did to catch you off guard.
 
Don't give up, it's refreshing to see such a positive attitude in a new player :D

Let this be proof that training mode and mission mode does absolutely nothing to the beginner that knows nothing of fighting games so everybody stop trying suggest noobs go to mission and training mode.  
 
The most ultra basic thing you can learn from just playing experience is am I safe/ can I punish?  
 
That is does doing this move leave me open for a free retaliation, and did the move my opponent did leave him open to punish. Yes you will look like an idiot spamming moves but if you can learn from matches and just refine when to use your moves then you would have already gotten better. I don't care if your only punish is doing a super, its still easy damage.  
 
Then after you realize through the fast pace gameplay the rhythm of when to attack and defend, go and learn combos and such in the training mode since you would actually have the skills to actually use the combos when you see an opening.  
 
Next step is making a synergistic team, but everybody has their own method to that.  
 
 
The only other tip I can say is no matter how many projectiles and stuff seem to be on the screen, always block the direction that your opponents main character is facing. Yes there is bound to be tricks like they teleport right above your head and you have to guess which way is he going to land, but that just the mindgames behind it all. 

I know for a good point man in a team.. I like to pick a guy with a decent amount of health and that does decent damage to.. nothing crazy..
 
For instance I usally use: Captain America or Ryu to start my fights there both really balanced fighters.
 
If you like bum rushing your oppenents right away I'd go with: Wolverine, Dante, or even X-23 tho she is a little light on health so if you get caught up use an assist or tag to get out of a jam.

Make sure you find assists that work with the combos you know how to do. If you get a sweet OTG combo with hulk you can nearly kill a character without even using any meter.

"assuming control "