V_PlateCarrier3_rgr is not included in arsenal regardless of which mode is used (add manually with function or with full arsenal).
User-made vests that inherit from it are also not included in the arsenal.
Wearing such a vest, opening the arsenal and selecting "vests" will remove the vest from the player.
Activate the arsenal with a the vest included (either by including all items or adding the vest specifically). Open the arsenal and see the vest isn't there.
Certain unassigned items are not present in the Virtual Arsenal.
https://community.bistudio.com/wiki/Arma_3_CfgWeapons_Equipment
The V_PlateCarrier2_rgr is an identical vest to the "3". I'm guessing that it is a prop for future addition, just like the Scientists suits were before the Karts release.
In SIX config browser they don't seem to have the same config values. When I'm home I'll check if they have the same model, but as far as I remember they don't.