Best way to trigger supplemental cooling fans when AC is on.

AC high pressure switch + side

That's a very simple way to get the job done, however you can trigger them to come on with the MS3 as well... but I like the kiss method on this one, don't make it harder than it needs to be.

Btw the ms3 does not deal with things like the ms2 does... you can shut the fans off when the car is at speed just like the OEM do, this can be done with the supplemental fans as well if the ms3 controls them. Or place the cooling fan circuit in series with the auxiliary fans, the ms3 can turn the fan on when the AC trigger happens... and the when the ms3 shuts the main fan off at speed the auxiliary fans shut off as well.

Fans can get in the way air flow wise when the car is at speed (above 40mph) and yes there are over temp overrides to turn the ac off and the fans on.

All this is done in the software.

It can even do trouble codes for you... like if the MAT or CLT sensors go out of range for instance it can trigger the check engine light if you set it up to do so....