How do the first three opening lines set the tones and theme in Beowulf?
This is a great question. It would be best to quote the first three lines. This translation come from one the great poets of our day, Seamus Heaney.
So. The Spear-Danes in days gone by
And the kings who ruled them had courage and greatness.
We have heard of those princes’ heroic campaigns.
These three lines introduce many themes. The words are suggestive of what is to follow - spear-Danes, kings, courage, greatness, and heroic campaigns. All of these words set an epic where a hero of great courage seeks greatness in a campaign of danger. The actions of Beowulf certainly fulfill these opening lines, as he fights the monster Grendel and then the dragon. By these actions, he finds greatness and glory.
The words, "We have heard" are also important, as epics tend to start as oral poetry, like the works of Homer's Iliad and Odyssey. Beowulf also started an a oral poem.