Three stamina curios do not add up properly. When you equip 3 2-stamina curios (6 total) you get 3 stamina bars on a weapon (tested on tactical axe mkIV goes from 4 bars to 7 bars), however having 2 3-stamina (6 total) only nets you two stamina bars (axe 4 bars to 6 bars)
Stamina blessing on curio will always give you +1 stamina regardless of what number it says. It’s a known bug.