I have seen a lot of Thompson, Snow and Mickens in person over the last 2 years. I would rank them in that order easily.
Thompson - This guy is the ideal Big 12 defender. Don't try to classify him as a particular position. He is best around the LOS. He would be a great spy against QBs who are running threats. He is big enough to set a hard edge against the run game and would cause receivers running short crossing routes to be taking their life in their hands.
Snow - Whether he wants to admit it or not he is a future LB wherever he goes. He is over 215 but should be playing at around 205. Another guy who does his best work around the LOS. He is a good kid who can be a good LB but needs to realize that.
Mickens - I don't want to bad mouth a high school kid but there is no way he is a top 10 in the state prospect. He played basically a slot corner when he played on defense. Unfortunately for him he is not athletic enough to do that in college at the highest level.Not a big hitter as a middle of the field safety either. He is a good high school player who, as others have mentioned, is a pretty typical Southlake maxed out kid. Did not play a single snap on defense in Southlake's last 2 playoff games