Complex sentence: Three patrols were engaged on road watch duties at any one time, with one watching the road for a week to 10 days, another would be en route to relieve them and the third was returning to Siwa after having been relieved.
Split into simpler sentences: Three patrols were engaged on road watch duties at any one time. One would be watching the road for a week to 10 days, another would be en route to relieve them and the third was returning to Siwa after having been relieved.