Rather than controlling the fan speeds with electronics, the airflow of two fans is obstructed by an adjustable air valve.
The speed can be reduced enough to make interesting pattern changes possible. The three fans are connected to a 9 volt block cell.
For two fans, three of the four mounting holes are countersunk at the outside, to make easy gliding of the air valve possible. Better do this by hand with a 5mm drill. A machine easily snaps through the material, damaging a couple of hours of printing. The third fan always runs at constant speed. The laser needs 3 volts.