[8] D3's first operations during the war were defensive, being ordered along with most of the rest of the flotilla to patrol in the east end of the English Channel to protect the British Expeditionary Force during its passage to France in early August.

D3 was deployed off the mouth of the Ems estuary, and while she spotted a trawler and a German torpedo boat during the patrol, was unable to get into a position to deliver an attack.

[14][15] On 28 June 1915, D3 was one of six submarines[b] ordered to patrol in the Bight in case of a suspected sortie of the German High Seas Fleet.

She arrived on station on the night of 10/11 February, and on surfacing the next day, spotted four light cruisers and four torpedo boats that had just passed over her, heading east, but they were already too far away to engage.

[20] In May 1916, it was decided to supplement the 8th Flotilla's normal operations in the German Bight with patrols off Terschelling on the Dutch coast.

[21] On 21 July, D3 left for another patrol off the Dutch coast, but was forced to return the next day when Arsine fumes from her batteries rendered her crew ill.[22] D3 was still listed as part of the 8th Submarine Flotilla in August 1916,[23] but by September, had transferred to the 3rd Submarine Flotilla, under the Rear Admiral Commanding East Coast of England, and no longer part of the Harwich Force.

In late March, D3, together with D7 and D8, supported by the depot ship Platypus were deployed to Lough Swilly for patrols to the north and west of Ireland.

[25] On 23 April 1917, D3 spotted the German submarine UC-66 stopping and sinking the British barque Arethusa off Eagle Island, County Mayo.

[26] Platypus's flotilla was ordered to Killybegs, on the Atlantic coast of County Donegal at the end of April 1917.

After sighting survivors in the water, and hearing their shouts, AT-0's commanding officer realised that a British submarine may have been sunk.

AT-0 dropped lifebelt and left to find a ship to rescue the survivors, but the men drowned before help could arrive.