It's 3 passives at a time. One for each slot (wep, ring/ammy, armor). You can destroy as many items and store their passives in the cube but you can only have a total of 3 active at a time.
Also the powers are extracted at their max value. So if you find a Furnace with 40% damage and break it down, the power stored in the cube is 50%
The slots are not weap, ring and ammy but weapon, armor and jewelry.