Hei there 
My third weapon slot is locked when selling items to the vendors.
I don’t know if this information will help as well, but I’m in the endgame, not all vendors haven been upgraded to the maximum yet.
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Thank you four your time and effort you are putting into bugfixing, it is a great game and I am enjoying it very much.
I also had this bug. I believe it was because I added my third weapon slot in another world besides my main world, with a secondary character. What initiated this permanent error, however, was my 3rd slot weapon equipped, a rusty sword, vanishing from my hand soon after during a bounty. i quit game, reloaded and unequipped weapon from third slot. however, i quickly found whatever weapon i did equip in the 3rd slot was invisible in the menu.
i quit again after unequipping everything, reloaded and then everything was visible in the personal menu, but vendors and chests/storage do not acknowledge that there is a third weapon slot. i can still rune and enchant the third weapon, but cannot store or sell it, unless previously unequipped.
one potential solution i will attempt to get the 3rd weapon spot back visible to vendors and storage, is to be totally unequipped and empty of any items the next time i level up w plague ichor. i will also expand my 3rd secondary item slot to see if this helps.
