What are the three main parts (plots) in the epic Beowulf?

Beowulf, in the tradition of much heroic poetry, consists of a "main" narrative and a number of short digressions, included to keep the audience's interest. The main narrative—that is, the story of Beowulf himself—can be divided, however, into three key sections or, as you say in your question, plots.
Each of these plots requires Beowulf to slay an enemy. First of all, he comes to the aid of Hrothgar and swears that he will put an end to the killings being wrought by Grendel, the monster. Beowulf is true to his word, and Grendel is slain. However, in killing Grendel, Beowulf has aroused the ire of Grendel's mother, arguably even more terrible than her son. The second "plot," then, culminates in Beowulf killing Grendel's mother and leaving, victorious, for his home land.
The third section of the story does not end so well for Beowulf. Partly in a bid to ensure his exploits will live on after he dies, the now-old Beowulf, who has become king, must slay a dragon that is terrorizing his people. However, he dies in the process, and the poem ends with Beowulf's people cremating his body.