Most RPGs give players an endless bag that operates as their inventory, letting them pull anything out and use it in combat, but that's not the case in Dragon Quest 3 Remake. Using items in combat requires some forethought, and while there is a bottomless bag, you can't access it during combat.
While you can use consumables in combat, there are also multiple weapons that can also be used as an item in combat to trigger special abilities or spells.
Here's how to use consumables and weapons as items in combat in Dragon Quest 3 Remake.
The item bag, which will be the majority of your inventory, can't be accessed while in combat. Each character instead has their own personal inventory which can be filled with items that can be used while in combat.
You'll first need to open the main menu and go to the items option, with the first few options being the party members' inventories.
To put items in a character's inventory, scroll down to the item bag option and select the item you want to give to a character. The second option below use is transfer, which will let you give that item to a party member.
Only the party member with that item in their inventory can use it while in combat.
To actually use an item in combat, select fight, then scroll to the right side of the next menu and select items. Any item in that inventory can be used during that character's turn to do the usual effects of that item during their turn in combat.
Most gear items only take up a slot, with a few exceptions.
Some items will have a bonus effect when used as an item in combat. When in combat and in your character's inventory, hover over the gear piece instead and select it.
This will have the character use that item and activate its effect free of cost of MP. Usually, these spells are weaker than those cast using MP, but the lack of cost may make them worthwhile when exploring long dungeons. Some items can be used indefinitely while in combat, with others having limited uses listed in the item's description such as the staff that casts Zin.