The Shulhan Aruch (Orah Haim 123:2) rules that after taking the three steps back one should wait in one’s place until the Kedusha, at which point one returns to where one prayed. The Gemara (Yoma 53b) compares someone who returns to one’s original place immediately rather than waiting for Kedusha to a dog who walks over its own vomit. Therefore, taking the three steps back is simply a means. The Rama (ibid.) adds that the Shaliah Tzibbur should wait the time it takes to walk four cubits (roughly 3 seconds) before returning to his place and starting the repetition of the Amida. Therefore, taking the three steps back can be viewed as a means to achieve the prerequisite time before starting the repetition. This is based on the Arizal’s explanation that the silent prayer and the repetition should be bound together, and this is done by taking the three steps back.
There are instances, such as on the High Holidays when the prayer is long, when one finishes one’s silent prayer and is not physically capable of standing until the Shaliah Tzibbur reaches the Kedusha. In such a case, one may sit down, but only after taking the three steps back. When one is ready to stand back up for the Kedusha, one should take three steps forward back to one’s original spot.
Summary: After taking three steps back, one must stand in place until the Kedusha, at which place one returns to one’s original spot. One may sit down after finishing the Amida but should stand up and take three steps forward for the Kedusha.