Three Mile Lake is located west of Highway 97, 4 kilometres south of Clinton. Physiographically it is located near the edge of the Cariboo Plateau. Annual precipitation averages between 300 and 400 millimetres (EMPR Paper 1991-1).
The lake is a semi-evaporitic playa lake located in the dry valley along the Ashcroft-Clinton road. Bedrock includes marine sedimentary (ribbon chert, limestone and argillite) and volcanic rocks (basic flows and tuffs) of the Permian to Upper Triassic Cache Creek Group; Jurassic sedimentary rocks (chert pebble conglomerate, greywacke, shale and grit); and non-marine sediments (shale, sandstone, tuff, diatomite, conglomerate and breccias) of the Miocene Deadman River Formation.
Three Mile Lake contains magnesium sulphate-rich brine which in earlier times was of much higher concentration than in 1937 (EMPR Bulletin 4, page 41). In 1937, the brine had a density of up to 1.008 and contained up to 0.66 per cent dissolved solids composed mainly of 74 per cent magnesium sulphate, 5 per cent sodium sulphate, 1.5 per cent sodium chloride, 7 per cent sodium carbonate and 12.5 per cent calcium sulphate.