Complex sentence: Once the first two spins align anti-parallel, the third one is frustrated because its two orientations (up or down) give the same energy-the third spin cannot simultaneously minimize its interactions with both of the other two.
Split into simpler sentences: Once the first two spins align anti-parallel, the third one is "frustrated" because its two possible orientations, up and down, give the same energy. The third spin cannot simultaneously minimize its interactions with both of the other two.