What three essential qualities does Bilbo have?

Characterization is created in aggregate—in the layering of traits to create a complete personality. If you take a character who is written as brave, intelligent, resolute, and determined, these qualities exist in conversation with each other and build on one another. In a way, I'd suggest, the personality is larger than the sum of its parts.
With that in mind, I would say the single most important element to Bilbo's characterization—and of the themes that are associated with Tolkien—is his normalcy. Tolkien really does tend to focus his eye on normal people with normal lives (though The Silmarillion is very much an exception to this). If we look at The Lord of the Rings, it's notable that, for all of its larger than life characters—Aragorn, Boromir, Legolas, etc.—it's ultimately Frodo and Samwise that the story hinges on (in most conventional heroic narratives, one would expect the hero to be someone like Aragorn).
The Hobbit is similar. Bilbo's a quieter sort of hero than your conventional heroic protagonist. He has no martial training. He's enlisted as a thief, but he's not had any experience in thievery. There's a humility in his background, and what he represents as a protagonist is an average person who's been swept up in a grand adventure. This represents a critical theme, not only in The Hobbit but also within Tolkien's larger body of work: the celebration of everyday people and the heroism ordinary people are capable of (in contrast to the acts of larger-than-life heroic personalities).
In addition, I'd note that Bilbo is brave. We see this in how he's willing to risk his life throughout the book, as well as in his willingness to embark on this kind of adventure (and to remain committed to it). While Bilbo is relatively average, he's more adventurous than most hobbits.
Finally, I would note that he's very resourceful when he has to be. He's a quick thinker, and he is quite skilled with words. We see this illustrated several times within the narrative: for example, in his riddle contest with Gollum and in his encounter with Smaug. One could also highlight the company's escape from the elves of Mirkwood as another example of Bilbo's ingenuity.
These are just a few of the qualities Bilbo displays throughout the book: while it is not an exhaustive list, these traits are essential to understanding Bilbo's character and his role in the story.