Watched Mathis the other day and I am not high on him at all. He is athletic for his size but not elite, probably 50th-60th percentile of NFL players. Doesn't anchor well, and as a 23 yo redshirt senior, he already has all the physical edge vs OL that he is ever going to get.
He reminds me a bit of Maliek Collins as a rotational-type 3 tech with little power. Collins has had a normal career for a 3rd rounder, but you wouldn't want to take him any earlier than we did. I'm not taking Mathis early either.
Also, I think looking for a 3T in this draft is barking up the wrong tree. I'm high on Gallimore and not looking to replace him as a starter... he can stop the run and rush the QB when healthy. I'm not sold on Osa, but he showed enough to where I would still give him snaps this season. Adding another 3T to try out just creates a logjam there. We need a 1T badly, and Mathis isn't it.