Eliot and Quentin realize that they have traveled back in time and spend the years working on the puzzle in episode 3x05 "A Life In The Day".
Margo locates Eliza in a bubble of time after reading Quentin's letter and learns where the third key is located in episode 3x05 "A Life In The Day".
Felicity, Thea, and their group head across the island looking for the plan Adrian Chase flew in on while Oliver, Nyssa, and Slade Wilson head the other way looking for the rest of the missing people. In a flashback, Oliver shoots down Konstantin Kovar's helicopter and fights him to the death in episode 5x23 "Lian Yu". The team at the airplane run to escape the explosions in episode 6x01 "Fallout".
Source: pursuit23 — Twitter