If they were located in the same place as our own Sun - at the centre of the Solar System - the stars would stretch out further than the orbit of Jupiter.
The red "supergiant" stars are more than 1.5 billion km across, pushing the previous record holder, Herschel's "Garnet Star", into fourth place.
The new research was presented on Monday at the American Astronomical Society meeting in San Diego, US.
The three enormous stars from the study are KW Sagitarii (9,800 light-years away), V354 Cephei (9,000 light-years away), and KY Cygni (5,200 light-years away).
Luminous and cool
Astronomers compared a sample of 74 red supergiants in the Milky Way.
The international team made new observations of the stars from the 2.1m (84 inch) telescope at Kitt Peak National Observatory in Arizona, and the 1.5m (60 inch) telescope at Cerro Tololo Inter-American Observatory, in Chile.
With close stars, scientists can calculate stellar sizes from their temperatures and luminosities.
Red supergiants are massive stars nearing the ends of their lifetimes. They are comparatively cool, luminous and very large.
The analysis also yielded the most accurate temperatures yet found for these objects. The temperatures of the coolest red supergiants are about 3,450K (3,177C), or about 10% warmer than previously thought.
"The significance of this study is that for the first time in many decades there is good agreement between the theory of how large and cool these stars should be, and how large and cool we actually observe them to be," explained Dr Philip Massey, the project's leader.
"For the past two decades, there has been a significant disagreement."
Dr Massey said that there was nothing wrong with the theory. Instead, the observations of the stars themselves had to be improved.
The study has been submitted for publication in the Astrophysical Journal.
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