At D3 level it just counts towards your yearly dates of competition, but it must be between or after seasons. I played a current D2 player last year in mixed who was rated 4.5 even though she was a starter on a nationally ranked team. She played in between seasons.
As for college players getting rated. My team has been trying to get a former D3 player who was never ranked in college nor on a nationally ranked team to play on our 4.5 team, yet it keeps spitting out 5.0 and appeals aren't granted. Meanwhile, at sectionals, we played a guy who currently is a 1st singles D1 player (who appealed his 5.0 rating) and a D1 commit who beat the other guy pretty badly. How those guys are able to play 4.5, but a non-ranked D3 guy isn't...just crazy?!
Ex-Coach: U of Delaware & Drew U. Pro Kennex Ki-5(R) & Gamma RZR 98 (L)
Dunlop Black Widow 17 (63lb) Babolat N.Vy 16 (64lb) USTA 4.5.