Note that the 3rd fan is ALWAYS supposed to be on. It comes on even before the lights.
The side fans can be tested by pretending to do a print - select PRINT, select anything, then go straight to TUNE menu and set fans to 100%.
Usually the problem with the side fans (and rear fan) is with the cabling above the print head. You do not need to take anything apart - just slide up the plastic mesh covering as Illuminarti says.
The red and black wire is for the fan on the back of the head. That needs to plug into the single receptacle that sticks up from the middle on the far side of the board. (Which will require you to unscrew the board from the frame to get at it). The blue and white cable pair is unused. The yellow green pair powers the side fans, so check that connection in your photo, and then go look at the other end of those wires above the print head.
Pull the sheathing back at the printhead end, and look at the connectors there. the yellow and green pair supplies the power to the two side fans. The yellow and green each go into a connector at the print head end, and a short green cable jumpers them together to wire the fans in series. The side fans connect to these two via their own red/black pairs. The red/black pair coming from the electronics board connects to a third red/black pair that goes to the rear fan.
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