User blog comment:Hawkeye2701/The Many Faces of Rei Ayanami/@comment-3517267-20130126235459

Pretty good summary.

But one thing about Rei III : her A.T. Field weakens because she already betrayed Gendo from the beginning by not taking her medicine which keep her A.T. Field from not collapsing.

Also, for Rei Q, she's apparently from an early batch of Rei.