There are actually four body slots if you include hands. You could have Chainmail (2 armor/bulky) and and a helmet (1 armor/slot), using three total slots, and a fourth for whatever weapon. Of course, if a player wanted to use two slots (e.g. for a bulky weapon) I'd let them; fictionally I'd say they would just carry less in their backpack because of the cumbersome nature of all that armor + weapons.
I guess the Cardinal Rule is: fiction first. If the PC was mutated (and sprouted an extra arm) I probably wouldn't give them extra slots, but I would let them wield three knives.
PS I kinda assume stuff like Plate Mail has a helmet attached (since it gives 3 armor). I'd even be OK with saying that Chainmail (2 armo/bulkyr) + helmet (1 armor/slot) sorta combine together, and use one backpack slot. Of course, different Wardens will rule as they see fit.
