for a project I want to apply two 120 mm 12V fans (not for computer cooling). The ones I got have only two wires and work always on full speed (about 90 cfm). The full speed is not enough for me, so I looked for fans with about 150 cfm (at full speed). All I found have three or four wires (eg. this one http://www.frozencpu.com/products/12944/fan-858/Sunon_120mm_x_25mm_Precision_Ball_Bearing_Fan_-1500_CFM_PMD1212PTB1-A.html?tl=g36c435s1106). Since I have already done the wiring and don't want to change, I wonder if I could use such fans (my Arduino only controls, wether the fans will run or not, i.e. wether they get power or not). At the end of my wiring solution I added a box for the remaining wires. If necessary I could add some electronics inside to make the fans running at full speed, but if possible I want to avoid to send control signals through the third or third and fourth wire.
There is no spec sheet on the fan you provided the link to. I suspect the fan runs on 12V so there is a 12V pin, and gnd pin. Then there is an optical encoder that needs 5V input and output pulses to tell you how fast it is running at.
If it is a standard computer-type fan, then the red wire is power (+12V) and black is ground. On a 3-wire fan, the third (usually yellow) wire is an open-collector output from the tacho. Leave it disconnected if you are not interested in measuring the fan speed. On a 4-wire fan, the 4th wire is the PWM input, used to control the fan speed. If you leave it disconnected, the fan runs at full speed.