Mark's three "wicked" barons, Godwin, Ganelon, and Denoalan, are the power behind the throne in the kingdom of Cornwall, and that's definitely not a good thing. When they become jealous of Tristan, they plot to expose his affair with Yseut. In fact, that's pretty much all the barons do in the story, which reflects badly on their characters. Where are they, for example, when Mark needs a champion against Morholt of Ireland? Or when Tristan offers to undergo trial-by-combat to prove his innocence? The barons talk a good talk, but they aren't willing to walk the walk by backing up their words with swords.
Arthur's knight, Evain, describes Denoalan as someone who "delights in making accusations" and who "knows how to make a fool out of the king." This description applies to all three of these dudes (14.125). Unfortunately for Tristan, though, the three barons are powerful. They threaten to wage war against him, and convince their "friends" (a code word for wartime allies) to do the same if he refuses to banish Tristan.
Mark obviously perceives the barons as a real threat, since he is quick to give in to them at every turn. The only way of solving the problem of the three barons is to get rid of them, something Tristan finally manages to do when he kills two he catches spying on him and Yseut. Yet even then, the barons' power continues to make itself felt, prompting Tristan to go into exile to avoid it.
One question we might ask is what three honorable barons might have done in this situation. How would the have shown their loyalty to Mark? Would they have told him about the affair? Would they have sided with Tristan? The three barons complicate the themes of betrayal and loyalty just as much as Tristan and Yseut do.