Chess Opening Theory/1. e4/1...c5/2. c3/2...Nf6/3. e5/3...Nd5

3...Nd5
This is obviously the only good move, as 3...Ne4?? 4.d3! forfeits the knight, whilst 3...Ng8? is way too passive and forfeits two tempi. White usually plays 4.d4 or 4.Nf3. If White played 4.Bc4, or 4.g3, Black is fine after e6 and eventually d6.