How did Dobby know that Harry Ron and hermione were in the cellar at the malfoy residence?

I'm not sure if you've read the books or not, but for the sake of this answer, I shall assume that you haven't. Anyways, they didn't show this in the movies but before Sirius was killed, he gave Harry a mirror. Sirius had a matching one and Harry could use it to "call" Sirius. It was meant as a way of secret communication since the corrupted Ministry was watching most forms of communication. When Sirius died, Harry broke the mirror in frustration. It broke into pieces at the bottom of his trunk. In the beginning of the seventh book, when Harry was cleaning out his trunk, he accidentally sliced his hand with a shard from the mirror. Later, he looked into the shard and saw an eye looking at him. The eye looked like it belonged to Albus Dumbledore. However, it quickly vanished. Since that incident, Harry carried the shard around with him. When they got trapped in the cellar, he took out the shard and asked for help. Suddenly, Dobby appeared. What he didn't know at the time is that Albus' brother, Aberforth somehow had Sirius' mirror that was used to communicate with Harry. So, when Harry asked for help, Aberforth heard it and sent Dobby to help Harry.