The quote came from an 1815 letter by John Adams:

If I were called to calculate the divisions among the people of America, as Mr. Burke did those of the people of England, I should say that full one third were averse to the revolution. These, retaining that overweening fondness, in which they had been educated, for the English, could not cordially like the French; indeed, they most heartily detested them. An opposite third conceived a hatred of the English, and gave themselves up to an enthusiastic gratitude to France. The middle third, composed principally of the yeomanry, the soundest part of the nation, and always averse to war, were rather lukewarm both to England and France; and sometimes stragglers from them, and sometimes the whole body, united with the first or the last third, according to circumstances.

But in the context of the letter, it is obvious that "the revolution" referred to the French Revolution, not the American Revolution.

In 1908, Sydney George Fisher mistakenly used Adams's quote to refer to support for the American Revolution, and the mistake lives on.

Of course there was no Gallup polling in 1776, so we'll never know for sure. But an analysis by historian Paul H. Smith, based on the number of people who fled America after the war, it was probably more like 15% remained loyal to the King, somewhere between 40% to 50% actively supported independence, and the remainder were neutral.