I have 3 Apple iMacs (27 inch, 2009).
Is it possible to use two of those iMacs as displays for the other iMac?
If so, how do I achieve this?
It’s possible to use an iMac as a secondary display of another iMac. You’ll only be able to use one of your ‘other’ iMac’s to do that.
To do so, you need to activate ‘Target Display Mode’ on the iMac you want to use as a display.
Your target iMac will now display the desktop of the other, connected Mac. To exit TDM, just press Command + F2 again. It will also exit if you disconnect the cable or shut down/sleep the other Mac.
Remark: this only works on iMacs up to 2014, The 5K Retina and newer ones don’t support this.
If you are referring to Target Display mode, then you have to read Apple's website Use your iMac as a display with Target Display Mode. The system requirements are repleated below.