Marmion Marine Park, declared in 1987 as Western Australia's first marine park, stretches about 20 km from Trigg Island to Burns Beach and protects more than 136 fish species, ten seagrass species and 21 seabird species. The park's reefs run in lines parallel to the coast, and Three Mile Reef is the outermost, roughly 3 nautical miles offshore. DBCA's park guide describes it, with Little Island, as an offshore reef with better visibility than the inshore sites.

The reef is limestone with ledges, overhangs, caves and swim-throughs, covered in kelp and red algae on top and sponges and sea fans in the shade. Divers report the species typical of Perth's offshore reefs: western blue devils, blue groper, buffalo bream, dhufish, pink snapper, samson fish, bullseyes, morwong and old wives, with western rock lobster and wobbegongs under the ledges and Port Jackson sharks in winter. Australian sea lions sometimes visit.

Because the reef lies well offshore, visibility is typically better than on the inshore reefs, but the site is exposed to swell and needs a calm morning. Perth operators such as Image Dive & Charters include Marmion's offshore reefs in weekend morning charters from Hillarys, returning before the sea breeze.

Three Mile Reef suits Open Water divers comfortable with boat diving, and it offers a good introduction to Perth's offshore limestone reefs without the longer run to Rottnest.

Published site-level detail, including depths and exact positions, is limited. The information here combines DBCA's description with general knowledge of the park's offshore reefs, so check depths on the sounder and ask the operator for the current dive plan.

Launch at Hillarys Boat Harbour (public ramps off Southside Drive) or the Ocean Reef boat ramp further north. Three Mile Reef is the offshore reef line roughly 3 nautical miles west of the coast, about 10–15 minutes by boat from Hillarys. Image Dive & Charters runs weekend morning trips from Southside Drive, Hillarys Boat Harbour (8 am departure, back by 10:30 am).

Private boats should anchor in sand beside the reef and keep a lookout for other vessels.

Marmion's reefs are exposed to south-westerly and westerly ocean swell and to the afternoon south-westerly sea breeze (the Fremantle Doctor), which in summer usually arrives between late morning and early afternoon. Three Mile Reef is the outermost reef of the park and fully open to the ocean, so dive in swell under about 1.5 m and before the sea breeze. Winter fronts bring north-westerly gales and big swell.

Visibility is usually better than at the inshore reefs, often 10 m or more.

Descend the anchor line to the reef, follow the main ledge or reef edge in one direction at 12–18 m, looking under overhangs and into caves, and turn on half your air to return along the reef top.

Image Dive & Charters – boat dives in Marmion Marine Park every Saturday and Sunday morning from Hillarys; other Perth dive shops run occasional trips. Also dived from private boats.