Simply refill them, yes.
yes
I have never seen nor heard of a revolver airsoft pistol with non reusable shells. You should have no problems reusing them.
Yes.
It depends on the reason why they are
just 3 shot shotgun shells
Yes, but check before hand to make sure they can handle the pressure of Propane (they can if they can handle Green Gas or Top Gas)
Airsoft gun grenade launchers utilize a shell that is loaded into the launcher. the shell is powed by a high pressure gas such as green gas or propane. BB's are loaded into holes placed around the head of the shell. (usually about 120 per shell) Then once the shell is loaded into the chamber and the trigger is activated, the gas forces the BB's out. It is more or less a one-shot high volume airsoft shotgun (unless you have multiple shells that is)
Yes, once you shoot then, take out the cartridge and refill it the same way you loaded it up the first time.
No.
No, its not a good idea to reuse, them because reused ones can have particles of dust and dirt that will inherently get picked up when fired. Its also a bad idea to use low quality BB's cause they tend to have a seam around them from molding, and both them and the re-used BB's can jam the gun. if you need to find some really inexpensive BB's that are really high quality, no seams, and come in high quantities, i get all of the ones that i use from www.airsplat.com. I really love them, they are really precise.
Any spent plastic shotgun shells (hulls) should be reusable, as long as the brass is re-sized before reloading. Paper isn't recommended, and brass casings are practically non-reloadable.
No, they can not, do not attempt to mix real ammunition in airsoft guns.