While I have played fighting games very casually for most of my life, before MvC3 I couldn't even reliably pull off a Dragon Punch motion, so I can tell you that tri-dashing was and is a pretty difficult thing for me to pull off, at least compared to how you see the pros doing it. Unfortunately, I think it's the kind of thing that takes a lot of practice to build up muscle memory and really be able to do consistently.
The fact that it requires two buttons to dash is also a challenge for me as I sometimes will hit one right before the other and will not get a dash. For a while I had a single button mapped to dash, but I decided it would be better in the long run if I broke that habit.
In any event, from what I can tell Doctor Doom is one of the least air dash intensive characters when comparing to someone like Magneto or even Storm. He actually uses a lot of dashes, but his loops and attack style don't seem to require them to be used quite so quickly or accurately.
I think the best thing to practice is probably just the transition from super jumping forward into dashing down. The trick with that is to make sure you get high enough with your jump that when dashing down and attacking, your attack still has enough time to come out before hitting the ground. That's an issue for me with Magneto. But just going from jumping into hitting those two dash buttons, all at the correct angles (UF, then DF), that's basically the foundation of it and it's something I try to practice. Perhaps some of the experts around here can provide more insight.
