My case has 6 fans.
3x120mm infront - intake
2x120mm top - exhaust
1x120mm back - exhaust
MB Temp is around 25/35 (B450 Tomahawk Max)
CPU Temp is around 30/40 idle (3600X OC'd - Arctic DUO)
GPU Temp is around 30/40 idle (MSI 1070 Ti Gaming OC'd)
Depends on your configuration, but I would put them to exhaust.