There is something I remember about how each of the sets of three (until the final single makkah killing the firstborn, which was equal in intensity to all the previous ones put together)represented the same onesh, but in greater increments. So Dam, Arov, and Barad represented one type of avlah that the Mitzrim were punished for committing, Tzefardeya, Dever, Arbeh, another type, and Kinim, Shechin, and Choshech, the last type. It might be that the first set represented a change in nature, the next three were an affliction of noxious creatures, and the last three were plagues that afflicted the body, including the darkness that was so thick that literally it paralyzed all movement. I know I am saying this incorrectly or weakly, so if anyone knows the correct D”T on this I would love to have it.