The formula with 3 substituted
miles = 3 ÷ 1.609344 = 1.864
The constant is an exact definition, not a rounded factor: the mile is 1.609344 km, the US gallon is 3.785411784 litres and the imperial gallon is 4.54609 litres. The Kilometres to Miles Converter explains where each comes from and gives a longer table.
At a walking pace of 5 km/h, 3 km takes about 36 minutes. At an easy running pace of 10 km/h it takes about 18. In metres it is 3,000.
What 3 kilometres means in practice
3 km is 1.86 miles, and the 3000 metres is a standard track event and the classic distance for the Cooper test's junior version. Walking it takes about 36 minutes; a steady runner needs 15 to 18.