"in" is the abbreviation for inches, and school maths/physics class taught us that length × length × length = volume, which is exactly what you have: 3 kilometres × 1 inch × 1 mile.

My physics lessons at secondary school must have spent at least a week on SI units, base and derived units:

  Force = mass × acceleration
      N =  kg  ×    m·s¯²
  (i.e. a Newton is a kilogram-metre-per-second-squared.)

  Pressure =    force ÷ area
     Pa    =      N   ÷  m²
     Pa    = kg·m·s¯² ÷ m²
     Pa    =   m¯¹·kg·s¯²
  (So Pascals are Newtons-per-square-metre, or kilograms-per-metre-per-second-squared.)

Try: units 3km miles