For standard timer chips like the 555, 3 minutes is a long time and 30 minutes is a loooooong time. So the logic core would be better as a clock and divider to get the 3 minute period and another counter to get the 30 minute overall cycle time. Or it all could be done in the smallest PIC. Beyond that, everything alec asks is needed to create a final design.
AND
When the fans are "switched off" in the middle of an ON phase, does the timer get reset back to the beginning of the on phase and held waiting for some kind of restart signal, set to the beginning of the OFF (27 minute) phase and left to run, or something else?
ak