Short and clear:
Can i upgrade my dwarves multiple times on one expedition?
One of my dvarves is on expedition. Other is doing something else, and is level for example 5. Expedition is of tier 3.
Can I get 3*[ upgrade all armed dwarves by 1], thus upgrading my other dwarf on level 8?
You can only select each thing on the expedition card once per expedition, so the answer is no just as it is no that you couldn't take 3 pumpkins.