You are going to find, on any given D3 team, a player (or a few) who are legit D1 player, who, for whatever reason, are playing at the D3 level. Just the same, you are going to find perhaps a player or two who should not be on a D1 roster. At D2...it is going to be in-between.
In terms of overall talent, you are going to find many more hard throwers at D1...and even comparing a good D3 team with an average D1...the D1 is going to have many more ATHLETES...players who are bigger, stronger, faster, and in better physical shape. That is my observation from working in D1 athletics for 16 years and my son playing on a nationally competitive D3.