Traditionally 3 trees have been planted in front of the main gates of the most of Korean palaces that represented 3 prime minister called as 영의정, 좌의정, 우의정. The reason why they were planted there is to keep monitoring the kings' ruling and governing the kingdom for preventing any misbehalves of the absolute power, the King.
정조 liked to imitate what his late father had done so much and oneday he heard from one of his ministries that his late father had planted 3 trees in front of 온양 행궁's main gate while he had been visiting there previously and he wanted to do th samething. so he ordered to plant these 3 trees there.
They are the only remainers nowadays. So we can not find 3 trees in front of any main gates of other palaces in Korea except 수원 화성 행궁.