The discovery was made using NASA's Spitzer Space Telescope.
All three planets circle their parent star at the right distance to potentially be habitable.
Scientists are thrilled by the discovery and say it is one step closer to finding life outside Earth.
There may not be life on Mars, but there could be on one of three newly discovered planets.
Scientists from NASA have discovered seven more earth-sized planets within the Milky Way, three of them containing what are considered some of the necessary conditions for life.
The planets are circling the star known as Trappist-1, within the Aquarius constellation, roughly 40 light years from Earth, close in space terms.
The associate administrator of the Science Mission Directorate at NASA, Thomas Zurbuchen, says one of the three planets may be a second Earth.
"What we really have in this story is a major step forward towards answering one of these very questions that are at the heart of so many of our philosophers of what we are thinking about when we're by ourselves, and that basically is, 'Are we alone out there?'"
But it is going to be a long time until scientists know whether there is life on any of the newly discovered planets.
There is much they do not know about the planets, including their atmospheric conditions.
Those would indicate the temperatures on the planets' surfaces.
But scientists are excited about the possibilities and say, because the planets are so close to Earth, they are perfect for further study.
"These planets are among the best of all the planets we know to follow up, to see, for example, for the James Webb Space Telescope that we're going to launch (next) year, the atmospheres, and also to look at bio signatures if there are any. The discovery gives us a hint that finding a second Earth is not just a matter of 'if' but 'when.'"
Scientists have been using the Spitzer telescope to measure the planets and their distances from Trappist-1.
An astronomer from the University of Leige in Belgium, Michael Gillion, says the measurements show promising signs of life existing.
"Well, we have measured with Spitzer very, very precisely the sizes. And, furthermore, we have, thanks to Spitzer, too, preliminary measurements of the masses for six of them. And, for one of them, our measurement is precise enough to strongly suggest a water-rich composition, which is very exciting because this is one of the planets in the habitable zone."
Never before have three planets been discovered in the habitable zone around a single star outside of Earth's solar system.
A professor of planetary science and physics at the US Massachusetts Institute of Technology, Sara Seager, says the discovery means scientists have three chances of finding life.
"Well, with this discovery, we've made a giant, accelerated leap forward in the search for habitable worlds and life on other worlds, potentially speaking, because, with not just one planet but several, we have room that, if we didn't have the habitable zone quite right or weren't sure quite what we're looking for, we have many chances over."
The next step will be to get a closer look at the three planets.
In 2018, NASA is set to launch the James Webb Space Telescope, which will have greater sensitivity than current telescopes.
The James Webb will be able to detect the chemical fingerprints of water, methane, oxygen and ozone, plus other components key to finding whether the planets are habitable.
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