During the 1920s, the HMS Argenta vessel was used as a military base and prison ship for the holding of Irish Republicans by the British government as part of their internment strategy after Bloody Sunday. Cloistered below decks in cages which held 50 internees, the prisoners were forced to use broken toilets which overflowed frequently into their communal area. Deprived of tables, the already weakened men ate off the floor, frequently succumbing to disease and illness as a result.  There were several hunger strikes, including a major strike involving upwards of 150 men in the winter of 1923. By February 1923, under the 1922 Special Powers Act the British were detaining 263 men on the Argenta, which was moored in Belfast Lough. This was supplemented with internment at other land based sites such as Larne workhouse, Belfast Prison and Derry Gaol. Together, both the ship and the workhouse alone held 542 men without trial at the highest internment population level during June 1923.
Question: How many other internment sites were listed as supplements?
Answer: 3