Well, you have to start at the beginning of recruiting and look at people on your D2 screen, and then notice when they move to your D3 screen.
There is no special way to search for these people, because each school sees different dropdowns. Like basically a A+ D3 school is going to see 1000 D3 recruits on their starting screen, but a C- D3 team will only see 600. Over time, recruits open their minds to lower level schools, so the A+ school will begin to see some more guys show up on their D3 screen. The C- school will too, but it will be poeple that the A+ school has seen a D3 since day 1.
Recruits start dropping about 24 hours into recruiting, with the most movement happening right around the start of signing. No one moves much the last day.
It can be worth signing a D3 player, it depends. You have to see what D2 players are open to dropping (call them, if they say "backup" they will drop likely) and then decide. The recruiting part of the game requires a lot of thinking to be good at it in my opinion. Hope this helps.