Everyone is pretty well balanced in MvC3, so there is less of a gap in terms of what's required to learn a character here than there are in most of the versus series. That said, you probably want to avoid Phoenix until you've got a handle on the game. (As she has by far the least health of anyone.) And probably Trish too, as she's very strategy oriented. You might consider a character like Dante for your point man though, as it seems most newer players and those genuinely unfamiliar with versus fighters have an easier time adapting to his moveset. Really, it's going to boil down to your preference in the end and whatever personal playstyle you find you enjoy the most. (And that which works best for you.)
Experimentation, in other words, is your best bet. What you want to try and do though, is find one main character you feel comfortable with and have them on point in most of your fights, with a secondary character you can fall onto if the first dies. (Ideally, having these two characters be good at different things.. that way you can adapt to different encounters better.) Your third character should probably be reserved mostly as an assist, though you're going to want to still be able to hold your own with this character in case of an emergency. A solid assist is incredibly important as you get deeper into the game.. Without a good assist, you could find yourself in some serious trouble as you face players of progressively higher skill.
My policy is usually this: One Ground or Ground\Air man on Point that does the majority of the damage, One Air man that can be tagged in or fallen back on to take point when necessary (this character also preferably has a tilt-up assist, or some kind of launcher), and one character that has a really good pressure assist that can distract\punish anyone on the ground whenever possible. (Though again, you need to be able to use this character if it comes down to it. So they can't just be used as an assist without you having no understanding of how to play as them.)
Storm acts as my point man personally, I use her to lay the beat-down on people on the ground and try to juggle them up into the air with repeated lightning-attack's. I use Morrigan as my air man, usually having Storm setup a combo that leads into Morrigan tagging in and finally following through with one of her hypers. (Darkness Illusion is wonderful.) I have Shadow Blade set as Morrigan's assist, as it makes a nice launcher and punishes anyone trying to dive on Storm from overhead. Storm herself is using Lightning Attack as her assist at the moment, which admittedly isn't the best.. I might start using Whirlwind instead, as that could prove a nice pushback for relieving pressure off of Morrigan from time to time. (Lightning Attack is quick though, it's just more a matter of trying to get that assist to connect.) Phoenix is on TK Overdrive for her assist, which I use as my pressure on the ground to keep an opponent locked down whenever possible.. It deals nice damage when it connects, and it's pretty fast. When forced to play Phoenix herself I vary it up between games of keep-away and raw brute force bull-headedness. (I charge in without fear, in other words.) I usually only pull Phoenix out when I have 5 Full Bars and can spare her death should it occur. I generally then go completely apeshit on my opponent as Dark Phoenix and destroy them in a flurry of fire. (This is also about the only time I ever use X-Factor.. It really makes her quite ridiculous.)
I rarely use hypers in my strategies unless I already have Dark Phoenix up, or if I already have a huge advantage over my enemy. Or.. if I vary it up and have Morrigan on her Dark Harmonizer assist, which gives you a quick 30% bonus to your hyper bar. (Can build up a bar so very quickly with that assist..) Ice Storm is is my hyper of choice when I find I have an opening in the middle of battle.. Otherwise, Darkness Illusion is what I always try to sneak into the middle of my combos. Aaaand.. Wow this post came out a lot longer than I was expecting it to.![]()