Numerous times where I tried to go for Ultra,I do shadow dp or another shadow attack for some reason? Any help?
Most of the time, this is the cause of a slight input error. Just go practice doing ultras in training mode and get the motion down as smooth as possible.
But the inputs are the exact same. Just one has 2 instead of 3 punches.
Are you using a controller or a stick? using LB is incredibly useful in assuring the ultra on a normal X1 controller as it is all three punches pushed at once.
That is exactly what I do.
Hmmmm… sorry, but I don’t think I can offer anymore help beyond this. 
I would blame a mix of repeated inputs and “negative edge;” I’ve had issues with that in the past myself
I would have x3 buttons set if this is really an issue for you. If it’s still messing up for you, then you probably pressed one of the buttons by mistake and it didn’t read the input right. idk lol
Negative edge. When you input the command with 3 attacks it detects it at a specific assortment in time.
I did it with the triple punch but sometimes shadow dp happens 
Certain characters have bugs that make them more likely to mess up the ultra input. Hisako and Raam have this problem I believe.
I bind 3x to a paddle on the elite which works well, though before I got it I found RT worked well for me.
You either missed the input timing and it registered as a dropped combo into Shadow DP, or you did the motion twice and pressed 3Ps somewhere between them. Least, that’s what I could think of.
If you are still having problems I’d recommend turning on ultras in combo assist, You can customise it so it’s ONLY ultras if you don’t want to use the other features. It simplifies it so its forward and all 3 punches for every character, so if shadow moves are turned off in the CA settings then it should activate every time.