The A3 begins by proceeding ahead at a crossroads in Lisburn city centre, where the A1 veers to the left. It continues through the eastern suburbs of Lisburn before it comes to Junction 9 of the M1, where it intersects with the A26 travelling southwards from Antrim. The A3 carries the course of the A26 through the villages of Moira and Magheralin, before the A26 branches off again upon entering Lurgan. The A3 continues through the town centre of Lurgan, and becomes dual carriageway from the outskirts of Lurgan through a series of roundabouts in what was planned as the new city of Craigavon. At "Central Roundabout", the A3 veers north-easterly onto "Northway", a single-carriageway by-pass of Portadown. "Northway" was originally planned to be an extension of the M12 into the centre of Craigavon, but like many parts of the planned town, it was not built. Close to Portadown town centre, the A4 road branches off and the A3 continues to the city ofArmagh, bypassing the village of Richhill. On entering Armagh, the road meets a series of other main roads leading to the city along a half-ring road; firstly the A51 from Gilford, then the A28 from Newry and the A29 from Dundalk. It then continues its route through the villages of Milford and Middletown before reaching the border with the Republic of Ireland in the townland of Ardgonnell. The road continues as the N12 to Monaghan and then the N54 road towards Cavan. The Monaghan–Cavan road resumes the A3 designation twice, where it enters southeast Fermanagh: first from the western outskirts of Clones for two miles, before reverting to the N54 in the Drummully salient of the republic; then again for another short stretch where it meets the B533 from Newtownbutler at a Y-junction at Wattlebridge before reverting again to the N54 until it meets the N3 just outside Cavan. Whereas the main A3 section from Lisburn to the border near Monaghan is a primary route, the two Fermanagh sections have secondary status. A 3 mile stretch of the A3 between Portadown and Richhill was dualled in 2014.
A link road through the northern and western areas of Armagh city, with a further link in the southwestern part of the city, construction timetable for after 2015
Improvements to the junctions on the Northway at Portadown; construction timetabled for after 2015