Here are some actions I found my experimentation/saw online that I couldn't find in the manuals/in-game help of C2/3. (more so since the steam version of Commandos 3 does not come with a manual.)
Control: Use last selected weapon. (can use this to use the Harpoon gun on land in C2.)
Shift and Left-Click in inventory: Take only one item from a stack (So if you have 3 rifles, you can give one of them to another commando.)
Right click on stacked item: (Sniper rifles and Disguises) Cycle stack (So if you take a mechanic unifrom and then a soldier's in the same slot, you'll switch to the soldier's uniform, will have to take off disguise if already wearing it to see the difference.)
The Green Beret can use the Anti-tank gun in the second Stalingrad mission in C3, it functions a bit like a tank from earlier games left click to move it and hold control to fire the gun.
The sapper can use Machine gun Tripods in Destination Berlin. (Not that they're useful, the too short range of fire-arms means any enemy can outrange him on the machine gun.)
While not quite a hidden action, it seems your commandos have different attack ranges/health values in Destination Berlin, this can be easiest seen with the Thief as he has a much lower firing range than the Green Beret/Spy in the Train level. (your range also seems to increase temporaily when already firing on an enemy but this isn't much help if you aren't already firing when an enemy moves into view.)
An manual I found online for C3, claims that if the Green Beret drops the LMG near a tripod, it wll re-attach so the sapper can use it, however to do this in the actual game you must hold Shift and left click on the tripod.
Edited by Samz, 11 December 2019 - 08:59 AM.