411 REFERENCE · Wikibooks
Custom Chess Theory/1.d4/1...d5/2.Nc3/2...Nf6/3.Nf3
Reference material by Wikibooks contributors. This retains its source identity and is not 411 news reporting.
3. Nf3 — Queen's Three Knights System
After 1. d4 d5 2. Nc3 Nf6, White plays 3. Nf3, developing the second knight.
3...Nc6 develops Black's remaining knight, giving the position a Queen's Four Knights System.
3...e5? is a mistake move Because its gives up a pawn you can't Get Back.