what type of relays do i need to control 3 three phase exhaust fans in a parkade,with 5 gas sensors,i have a common control circuit with one switch leg from each sensor all at one central location

 

We don't know what voltage or power requirements your fans have except that you mention 3-phase.

We don't know what your gas sensor output is.

Isn't "parkade" Canadian for a multilevel car parking ramp/garage?

You really need to consult a licensed electrician, as they have the knowledge and qualifications to do the job properly and safely. If wiring is done by amateurs, there very well may be fire/shock hazards created.