Is the aforementioned advertisement—which stipulates bringing 18 subscribers to obtain a free computer, and its owner claims to have acquired three devices by the same method—considered legitimate earning or does it fall under the category of forbidden (haram)?

It is permissible for a person to take the device in exchange for publishing a permissible advertisement for the company. This falls under the category of ju'ala (commission/reward). Although gharar (uncertainty) exists in ju'ala contracts, the jahala (ignorance/unspecifity) in the work involved does not cause harm, due to the need for it.