May I ask why? I've never seen a tire narrower than 205 for a 17" rim?

Otherwise you might be able to go 195 assuming you can find a matching profile.
The narrowest OEM size is 195/65R15 and that's assuming you have a non-2.3L engine (those came with larger front brakes that need a 16" rim for clearance). The OEM 16" specs are 205/55R16.
You can check with the people at BCMazda3 for more details.