Complex sentence: Three were stationed off Concepción while the fourth was sent to Juan Fernandez but gave up waiting in early June concluding that Anson's ships had either been lost at sea or had gone elsewhere.
Split into simpler sentences: Three were stationed off Concepción while the fourth was sent to Juan Fernandez. They gave up waiting in early June, concluding that Anson's ships had either been lost at sea or had gone elsewhere.