Shoma Muto who had worked on Character Design, Illustrations, and StoryBoard for Kamen Rider Series is in charge of Ultraman Trigger Dark Giants' Character Designs. Shoma Muto had announced this on his Twitter along with the above illustration of the Three Dark Giants.
For those who are unaware of Shoma Muto's works. He had recently provided the on-screen illustrations for Kamen Rider Saber one of which includes Yuri holding Kurayami along with Saikou. He was also the one who had designed the early versions of Humagears.
You might also remember these Artworks of Another Cross-Z, Another Mach & Another Chalice which he did two years back.