How do I change where an item is held when in 3rd person

You would need the handle part in the image to be where handles for pickaxes and swords are and that would make it difficult to fit a gun in that small frame, you would probably be better of making a 3d model gun with blockbench where you can change the posision of the model.

You can also manually change how an object is rendered. If you lock the code of an item, you should be able to open the file that handles rendering. Inside, you can change the position, rotation, and scale of the object in third and first person.

Hey, this is a old post but there's a very simple way. Search for the item file you want to modify: Just go to assets file in your mod files, go to models, select items, and find the .json of the item you want to modify. Once found go to blockbench and open this .json file (in blockbench click file>open file> go to the same file you found your item .json and open it). Now go to Display (Up right corner) and you can change how the item is displayed in pretty much every way. Cntrl + S to save the model directly on the mod file and it should be good to go.