DO NOT ATTEMPT TO FIRE 9x18 ROUNDS IN ANY GUN CHAMBERED FOR 9x19!!
The bullet diameter of a 9x18 round is .363-.365", while the diameter of a 9x19 round is .355". You will create significantly higher chamber pressures by attempting such a foolish stunt.
Regarding the firing of .380 in any firearm chambered for 9x18: Here you have the opposite situation with the 9x18 round having a bullet diameter of .363-.365" vs the .380 with a bullet diam. of ca. .354-.355". In this case, the .380 bullet is significantly undersized by .010" or so and can literally fall through the bore of a 9x18 firearm. While it may work from a mechanical aspect, the practice is totally pointless and definitely NOT recommended.
Cartridge interchangeability myths have been around for decades, mostly to help unscrupulous dealers to sell "turkeys" for which ammo was somewhat hard to come by. One instance that comes to mind is the old myth about the Astra 400 being "intended" to shoot a wide variety of 9mm cartridges when, in fact, it was only intended to shoot the 9mm Largo.
When the 400 was first sold in large quantities as surplus in the U.S., 9mm Largo was difficult to come by in any significant amounts, so the importers perpetuated an old myth that the gun was also intended to shoot .38 auto, .38 Super (!), 9mm parabellum, 9mm Browning Long and .380, all of which was a very dangerous practice.
Use only the cartridges for which the arm is intended.