Camels
Camels: Camels could survive the harsher weather and different terrain then horses and donkeys. The camels were also more used to carrying more awkward and heaver objects then horses and donkeys. Since camels didn't need to drink a ton of water then they could travel farther without stopping.
Donkeys
Donkeys: Since donkeys aren't as strong as camels and horses then they had to carry lighter things. They probably carried the food and water.
