Hello @axilirate.zero ,Welcome to the forums!
Currently, it is not possible to directly modify the loot pool of regular chests such as the Chest Mark III in UEFN, since the loot system for these chests is handled internally by Fortnite and there is no official option to customize it from the device itself.
What you can do instead is create your own custom chest and define exactly which loot you want it to drop. This gives you full control over the items being spawned, which sounds like what you are looking for.
Here are some tutorials that show different ways to create custom chests with custom loot in UEFN.
Creating CUSTOM LOOT in Fortnite Creative/UEFN
Hope it helps!