A greatly misunderstood king. Started his reign by winning the seven year war (sometimes called the First World War) against France. Gave the control of the crown estates (The Monarchs main source of income) to Parliament. His Governments repealed the Stamp duty imposed on Americans shortly after they were imposed. He always yielded to his cabinet in keeping with the role of a Constitutional Monarch. He is often given the blame for losing the colonies but that responsibility should be given to Lord North the Prime Minister who ignored petitions for representation given to parliament.
He had a great interest in the Sciences funding a significant collection of mathematical instruments now on display in London’s science Museum, he funded the largest telescope ever built at that time which discovered the planet Uranus (initially it was named after George). Helped the Agricultural revolution reach its peak.
Yet all he is remembered for is that in his last years 1811-1820 he went mad. 9 years of madness out of 59 years of reign isn't too bad.
Pedantic person: Don’t blame the King you should blame Lord North
So apart from total unopposed control of India; £8.1 Bn of estates; keeping Napoleon at bay; an enormous collection of scientific instruments; the discovery of Uranus; helping to maintain a system of democratically elected governments; massive advances in agriculture paving the way for the industrial revolution. What has King George the Third ever done for us?