three-day journey
A journey or distance that takes three days. Often used in classical or poetic contexts, but still understandable in modern Japanese.
It's a three-day journey from here to Kyoto.
Compound of 三日 (みっか, 'three days') and 路 (じ, 'road' or 'journey'). The reading みっかじ follows regular rendaku voicing of ち to じ.