in the combo begiining style(tai chi) of yeah you start pressing triangle
3 times fast then in the middle combo(judo) you start medium speed then
in the last part of the weapon (katana) you start at any speed but not slow
I suggest that if you havent done it yet, go do a few side missions, play a bit of arcade or puzzle, then go back to it. Sometimes the more you do it in a row the worse you get at it due to the overwhelming frustration.