For most of its length, SH 3 parallels I-45 and runs alongside the former Galveston, Houston and Henderson Railroad lines.

The first junction in Harris County is with NASA Road 1, which provides access to the Lyndon B. Johnson Space Center.

The highway continues to the northwest to an intersection with FM 2351 and passes along the western boundary of Ellington Field.

The highway continues to the northwest through South Houston until it reaches an intersection with Winkler Drive.

The highway turns to the west and follows Winkler Drive to its northern terminus at an interchange with I-45.

[9] On October 8, 1917, the routing had changed slightly, now veering further north from La Grange to Bastrop.

[12] On March 18, 1919, a split travelling both southwest to Kenedy and south to Goliad was added.

State Highway 3-C was designated on March 17, 1919 as a proposed spur off of SH 3A, splitting at Fayetteville and traveling northeast through Bellville and Hempstead to Houston.

[15] State Highway 3-D was designated on August 21, 1922 as a spur that connected the main route and SH 3A between La Grange and Columbus.

[11] State Highway 3-E was another spur from Uvalde to La Pryor designated on July 16, 1923.