It remembers the leaders of the Cornish Rebellion of 1497. Michael Joseph, a blacksmith (An Gof) and a lawyer, Thomas Flaman.
Astonishingly there was a battle near this spot between a rebel army from Cornwall and the army of King Henry V11. The rebellion was sparked off by taxes being levied in breach of traditional rights and a 10,000 strong army marched all the way from Cornwall to demand the King changes his mind.
He didn't and his 25,000 army with heavy artillery routed the rebels.
The leaders were executed at Tyburn (now site of Marble Arch).
Is this a precursor to the "No taxation without Representation" of the American Revolution?
It is amazing what history you just happen to stumble across.