The line was opened in August 1936 between Sol and Embajadores, a few days before the beginning of the Spanish Civil War. In 1941 it was extended from Sol to Argüelles, in 1949 from Embajadores to Delicias, in 1951 from Delicias to Legazpi, in 1963 from Argüelles to Moncloa.
In 2007, the platforms were lengthened to 90 metres (300 ft) to allow for 6-car trains of CAF class 3000. During this process, all of the stations were modernized and rebuilt, thus giving the original section of Line 3 a brand new look. Moncloa station was completely rebuilt adjacent to that of the Line 6, proving an easy connection between the two lines.
On 21 April 2007, the line was extended from Legazpi to Villaverde Alto, doubling its length. This was the first extension of the line since 1951. Further extensions on both ends of the line have been proposed. In the south, the line will be extended to El Casar station, where it will meet Line 12 and Cercanías Line C-3.[1]
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