There is no single flag of Saint Helena, Ascension and Tristan da Cunha. The territory is one administrative unit spread across three groups of islands in the South Atlantic, and each has a flag of its own, adopted decades apart.
Three flags, three dates
Saint Helena's came first, on October 4, 1984. Tristan da Cunha followed on October 20, 2002. Ascension Island was granted its own on May 11, 2013.
All three use the same formula, the British Blue Ensign: a blue field with the Union Jack in the canton and the island's badge on the fly, the shield in Saint Helena's case and the full coat of arms in the other two. None of the islands had arms of its own to begin with. Saint Helena's were granted by royal warrant in January 1984 and its flag followed that October; Ascension used the United Kingdom's arms until its own were granted in 2012, with the flag a year later. Each grant tracks the territory's slow move from colonial possession to a place with a recognized identity of its own.
What is on each shield
Saint Helena's shield carries a three-masted sailing ship flying the Cross of St George, approaching the island's rocky coastline, with the wirebird, the island's national bird, on a yellow field above.
Ascension's arms put a green chevron for Green Mountain and three white terns over a ground of blue and white waves, with two green turtles as supporters and a sailing ship as the crest. The turtles nest on Ascension's beaches and the terns breed there in enormous numbers, which is most of what the island was known for before the airfield.
Tristan da Cunha's shield holds four albatrosses, with a Tristan longboat above it and two rock lobsters flanking it. The albatrosses point to the island's seabirds, the lobsters and the longboat to fishing and seafaring.
Blue and white dominate all three, colors normally read as maritime and peaceful.
Flying them
Each flag flies on its own islands, at government buildings, ports, and official residences, and at local government sessions and celebrations. Protocol follows British custom, with local additions for island holidays. Tristan da Cunha flies its flag prominently on the annual Day of the Islands.
The Blue Ensign family
Nearly every British Overseas Territory flag works this way: Union Jack in the canton, local badge on the fly. It is a template, and all the interest is in what each territory chose to put in the badge. On these three it is wildlife and ships, which is a fair account of how much island life here has depended on the sea.
References
Tristan da Cunha Official Website