The three forms of initiation (Sen no Sangensoku) are based on the idea that an attack is the best form of defense. In Wado, we must always think attack even when the opponent is in the act of attacking. It is in the best interest to end the fight as quickly as possible. These priciples deal greatly with distance and timing and determining your opponent's mind and willingness to attack.
Go no Sen - Go no sen is block and attacking the opponent after their attack has finished. For example, an opponent throws a reverse punch at the face, the attack is stopped with a block and then follow up with a counter attack.
Sen no Sen - Sen no sen is a block and counter attack while the opponent is attacking. For example, an opponent shifts in with a reverse punch and at the same time, the karate-ka would shift back, block his reverse punch with an outward block and counter attack with a reverse punch at the same time. The timing here is very important because both people are committing to the attack at the same time.
Sen Sen no Sen - The concept behind Sen Sen no Sen is you sense your opponent's attack coming and you counter attack before the opponent can fully start his offensive attack. For example, both opponents are either shifting or standing in kamae and your opponent goes to throw a rear leg roundhouse kick, before he can get the roundhouse kick off, you shift in and jab.