Yes, installing alternative Operating Systems is possible, once made available. We are looking forward to the respective communities to port their Operating Systems — such as Ubuntu Touch, Lineage OS, Sailfish OS or /e/ — to Fairphone 3.
All Fairphone 3’s are shipped with the bootloader locked to ensure an attacker cannot compromise the device by installing their own system or boot image. If you decide to try an alternative Operating System or to contribute to the porting efforts, you can unlock the bootloader of your Fairphone 3 by following this step-by-step guide.