| Box Width: | 350px |
| Text Color: | white |
| Line Name: | Line 3 |
| Network: | Paris Métro |
| Locale: | Levallois-Perret, Paris, Bagnolet |
| Terminus: | Pont de Levallois–Bécon Gallieni |
| Operator Org: | RATP |
| Conducting System: | Conductor (OCTYS) |
| Rolling Stock: | MF 67 (47 trains as of 31 July 2023) |
| Number Stations: | 25 |
| Length: | 11.665 |
| Electrification: | 750 V DC third rail |
| Dist Between Stations: | 486 |
| Passengers Per Year: | 101.4 million including Line 3bis |
| Traffic Rank: | 10th/16 (2017) |
Paris Métro Line 3 (French: Ligne 3 du métro de Paris) is one of the sixteen lines of the Paris Métro. It connects Pont de Levallois–Bécon station in the near northwestern suburbs to Gallieni in the east. After opening as the network's third line in 1904, it was the subject of a number of extensions, with a major restructuring occurring in the eastern section in 1971, with an extension to Gallieni and the conversion of the original terminal branch to Line 3bis.
With a length of 11.7abbr=onNaNabbr=on, Line 3 crosses Paris from west to east completely on the Rive Droite, serving the residential areas of the 17th arrondissement, the Gare Saint-Lazare, important stores and shopping centres, the area around the Place de l'Opéra, as well as the east of the city, around République station. In 2017, it carried 101.4 million riders, making it the tenth busiest line of the Métro network.
The infrastructure works for Line 3 were auctioned off in six sections on 24 May 1902. The concession was granted to the CMP by the municipal government of Paris on 13 March 1903, but the declaration of public utility was only granted on 26 February 1907.
The works were rendered difficult due to the necessary displacement of existing underground infrastructure such as water, gas, and electricity lines, but also because the Line 3 was to cross a number of Métro lines, particularly at Opéra, where a special pit was built to house the intersection of Lines 3, 7, and 8. This masonry pit was 20m (70feet) and constructed fully so as to avoid any problems when building subsequent lines. Since the work was situated at the water table, it required concrete pillars made by sinking caissons with compressed air. The work lasted eleven months, from March 1903 to February 1904.
Another difficult point of construction was the crossing of the Canal Saint-Martin. To build underneath it, the canal was dried and its vault suspended. In the area of the Place Gambetta, because of the instability of the subsoil, which consisted of waterlogged sand balls, made it necessary to drain the soil, a very delicate operation considering the number of buildings that could be destabilised. Then the side walls were built from masonry shafts that are dark from the outside.
Although no funded plans for an extension of Line 3 currently exist, Line 3bis may be merged with Line 7bis to form a new line, possibly named Line 20, which Line 3 would then connect to. A western extension of Line 3 to has also been proposed.
Line 3 passes near several places of interest :