Where is it written in either the rules or the casebook that 3 feet touching the ground is all that is needed to establish a catch?
Of course it isn't all that's needed. There's such a thing as the 3-part catch process. It's how officials determine a catch has been made. The first 2 parts were (and still are) control of the ball and two feet down. In 2014, part 3 of the catch process required that the player hold onto the ball long enough "to perform any act common to the game." This is popularly known as the football move. In simple terms, the football move was the thing that the receiver did that showed he wasn't still trying to catch the ball. And it
had to come after he had control of the ball and two feet on the ground.
Informed discussion of the Dez play inevitably centers on the football move. That's because we know Dez completed the first two parts of the catch process (control + two feet down). Not even Blandino questioned that part. We know Blandino saw Dez get control of the ball and two feet down, because Blandino said
he looked for a football move. You wouldn't have any need to look for part 3 of the process if parts 1 and 2 hadn't been completed.
Where is it written that this overrides the "going to the ground" rule?
Item 1 (popularly known as the "going to the ground" rule) applies when a player goes to the ground in the act of catching a pass. It does not apply to players who have completed the catch process, just to those who are still in the act. We know that in 2014, a player could make a football move while falling because there was nothing in the rules about a player having to be "upright" in order to become a runner. (That didn't get added until the next year.)
Besides the rule itself, we also have casebook examples of players completing the catch process while falling, video explanations by Blandino of similar plays which show players completing the process while falling, and Blandino's own statements when asked why he overturned the catch. He said Dez's reach wasn't obvious enough to be considered a football move, because he didn't "extend the ball, or reach with two hands." If you can't complete the catch process while falling, it wouldn't matter how many hands you used or how far you extended the ball, so that's Blandino telling us that it's possible for a player to complete the catch process while falling.