You can use this thread for that: Official and Only Team Building Thread (ALL TEAM BUILDING TOPICS GO HERE)
But since you already made your own thread, I’ll answer you here
MVC3 is not a very execution heavy game, so when you see someone talking about a character taking a lot of execution, we’re talking in this game’s standards, so if you played a game with hard execution before, you don’t have to worry at all, just stick with the characters you like and you will adapt in no time.
Also, even the execution heavy characters can be used in an effective way without the execution heavy stuff (minus Viper), you didn’t need to know the ROM to be an effective Magneto in vanilla, you could just use the standard fly combo into DHC glitch and it would kill a character anyway, so just stick to the basics with the characters you like and also hit the lab to figure out the more advanced stuff.
About synergy, you just have to think about the team you’re trying to do and see what can each character do to help the others, the more they can do for each other, the more synergy the team has, it will also help you to set up your team order, like when you think about Wesker and Doom and see what each character do to the other you will come to the conclusion that Wesker benefits a lot more from having Doom’s assist than the opposite. The only exception would be if you like Wesker better as Dark Wesker, which is already a strategy of it’s own and does not require teamwork lol