Out of the trio, Regirock is the most polished in terms of design. Overall, none of them are fully realized. It would have been nice if you had been able to submit an updated version to see where you could have taken these. I have to go on what you have here. As they stand, they look very similar to the actual design of the trio.
Your Regice is pretty cool. I like where you went with the design with an overcoat that resembles something an eskimo would wear. Registeel is almost as cool, but I kept help but envision its torso as intestines with how they are coiled. All of these can be summed up by your Regirock, creative design, but somewhat lacking in closer details. You didn't make them totally conform to their elemental type, which makes them stand out from the actual designs.
Tauros. The direction you went by making each one of them resemble a bull is held back by the respect you pay to the original design. I would have really liked to see each of these on all fours with each of their types featured more subtly across each design. They look a little awkward being bipedal. You do have cool new symbols for each of them and I like how each of them are not carbon copies of each other. It is clear you wanted to differentiate each one.
I view your submissions as the more fully realized version of what Kirby was trying to achieve. The Registeel is very cool how you have gears for the hip socket and metal grooves to define facial features. Regice as a snowman is cool, but I think you could have made it more imposing by shaping the body in a more dramatic way and replacing the sticks with icicles for arms. Regirock lacks the same sophistication you have in the other ones. I really like how you transformed the trio into being more organic. That's one of the design qualities that appeals to me in legendaries.
These are really polished. The first thing that stands out to me is how you substituted the standard dots for helmet customization. Not only does it work well with your knight theme, but it also allows you to play up their type while staying true to the originals. The armor for each of your designs appropriately distinguishes between each of the trio. The only thing I can criticize is how they play out much the same way as the original designs. While they might not be the most typical looking Pokemon, they were well done in their execution.
Out of your submissions, I think I like Regirock the most. It is not easy to distinguish what they are based. The only thing I can make out is the typing of each one. The bright red is harsh in contrast to the natural color of each type. It would have been nice if you could have fleshed out your concepts more. As it stands they appear to be incomplete.