the exercise examples in the quote pasted are to improve your reactive ability.... You can do low level depth jumps but you dont need them right now.
Youre probably making your single leg jumping form worse by trying to concentrate so much on the penultimate step, instead of just jumping. Different athletes use waaaaaaaaaaaaay different penultimate steps, structure, leverages, speed, strength, all individual characteristics factor into that.
Practice the 1-2-3 drill over and over until you feel very comfortable with it, its just basically 3 steps accelerating into a jump on the third, you will naturally develop a "penultimate" step by doing this, just like you will naturally develop a plant by doing it with two legs. Main thing is ACCELERATE. Each step gets progressively faster, dont worry about the distance.