So Critical chance is its own frame for each Hime and each critical chance is a separate proc that adds the bonus damage to the same critical frame. In your example each Hime (and your Soul) will have three independent 10% chances for an additional 20% damage. Your soul (if you have the Crit rate ability) will also have it's own chance for an additional 10% damage. For any Hime (and Soul) you can have 1, 2 or all 3 weapon effects trigger for potentially 20%, 40%, or 60% additional damage. Since they are independent, you'll see less variability in damage the more sources you have, but there is always some variability.
For a single medium source of stinger at skill level 20, this will give you a 13% chance (base 3% + 0.5% per level) for 20% additional damage. This is equivalent to approximately an additional 2.6% (.13x.20) average damage over a large number of attacks. It is in its own frame so the more attack you have the more relative value you get out of it versus a standard assault bonus as you get a higher base assault. If you have +100% assault, this works out to about 5.2% additional average damage for that stinger class.
Lastly, please remember you need elemental advantage for crit chance to proc (This is the 'weak point!' text on the damage text for a hit).
Dejnov.