Contactor current ratings are always per pole.
For example a contactor rated at 40 amps AC3 could switch up to 40 amps AC3 on each pole.
For a 3 phase load, you can have up to 40A on phase 1 plus up to 40A on phase 2 plus up to 40A on phase 3 and the contactor will still be a 40A contactor.
Basically, regarding the contactor ratings, you will never sum the current on the 3 phases.
For example a contactor rated at 40 amps AC3 could switch up to 40 amps AC3 on each pole.
For a 3 phase load, you can have up to 40A on phase 1 plus up to 40A on phase 2 plus up to 40A on phase 3 and the contactor will still be a 40A contactor.
Basically, regarding the contactor ratings, you will never sum the current on the 3 phases.