Weapons don't stack with other weapons. The basic crit level comes from your BOFF crew, skills, fleet tac console etc, the critH for the weapon is then adds that weapon's cH and other situational modifiers like your accuracy minus their defense.
Your base critH and critD levels can be seen when orbiting a planet, spacedock, ds9 etc.
CrtH modifiers on consoles/other ship gear affect your ships overall chance to crit, and would increase the 3% figure.
CrtH modifiers on ship weapons only affect the critical hit chance for THAT particular weapon to crit. So if your base crit chance is 3%, and all your weapons have CrtHx3 - they each would have 9% chance to crit.
The character stats, tootltips, character/boff info sections and doff descriptions do have a poor reputation amongst players, sometimes its tough to see what a thing is actually doing.
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Your base critH and critD levels can be seen when orbiting a planet, spacedock, ds9 etc.
or does it add 3% crit and then 45% on top of it? or 45% of 3% on top of the 3%?
CrtH modifiers on ship weapons only affect the critical hit chance for THAT particular weapon to crit. So if your base crit chance is 3%, and all your weapons have CrtHx3 - they each would have 9% chance to crit.