And there is one more thing you could try.
1200 rpm for a 3000 rpm fan is about 5V.
So check if your fans will start and run using the red 5V wire instead of the yellow 12V wire from the PSU.
If they start on the 5V wire, you could get a SPDT on/on switch like this
to flip from 12V and 5V.
That will only give you two speeds but for 2$, ya can't beat that price.
But you have to make sure the fans can start on 5V first.
And if you want to install two switches instead, I can show you how to get 5V/7V/12V as well.