Telephones, fax machines, and electronic mail transmit messages at many businesses.
The subject of a sentence is the noun that is doing the action. For instance, in the following sentence “boy” is the subject: The boy kicked the ball. A sentence, however, can have more than one subject, and this will be indicated usually with the conjunction “and.” When a sentence has more than one subject performing the same action, this is called a compound subject as in The boy and girl laughed. With that in mind, we can see that the final sentence of the sentences you provided has three subjects (telephones, fax machines, and electronic mail) that are performing the action “transmit.”