Ichigo does not have a third bankai (nor does any other Shinigami). He has one and only one - Tensa Zangetsu. Tensa Zangetsu, like many bankai, has different techniques, but they are still part of the same bankai. Ichigo has Getsuga Tensho, which can be used in a number of different ways (launched at a target, held in the blade while attacking) and the Final Getsuga Tensho (where Ichigo becomes the Getsuga itself). He has no other known forms/techniques in Bankai.