One fascinating aspect of Seuthes III, which few other rulers here except Philip II can boast, is that his tomb has been found. One can visit it today in Bulgaria and see his original sword and a freaky bronze head of him that strongly resembles that on his coins.
Seuthes was a major thorn in the side of Lysimachos during much of the latter’s reign. They fought against each other multiple times. The first, soon after Lysimachos took over his then satrapy after the death of Alexander, resulted in roughly a draw. The second occurred much later and resulted in a Lysimachos victory, but he was unable to take advantage of it due to an assault by Antigonos I Monophthalmos.
Although Seuthes may have had to pay tribute to Lysimachos, he remained in power after Lysimachos‘ death. There’s some evidence that he was still in power even at the death of Kassander. During this time, he founded his own city that he conveniently named Seuthopolis (naming your own cities was all the rage back then), and arranged for his impressive burial mound.
That in itself should stand for something. Of all his contemporaries, he left the most evidence for us.