Eurolink, the venture company set up by SIAC and Cintra, beat off competition from a consortium featuring toll road operator NTR, civil engineering group Ascon and Portuguese road specialist Dragados, for the project.
The contract will be formally awarded on completion of detailed talks between the National Roads Authority (NRA) and Eurolink, which are expected to take a number of months.
It is the second time SIAC and Cintra have tasted success in the programme of toll roads that was launched under the National Development Plan in 2000.
The companies were part of an earlier group that scooped the Kilcock/Kinnegad bypass that will form part of the Dublin-Galway road.
NTR's consortium, the Celtic Roads Group, was named the runner-up in the bidding process and will be invited to step in if the NRA and Eurolink fail to agree terms.
However, it is unusual in project finance circles for a preferred bidder to walk away from a contract once talks have begun. The NRA received six bids for the project when it began the tender process in 2002, but narrowed the field down to four.
Detailed bids were then submitted by Eurolink and Celtic Roads, together with Icon, a group featuring PJ Hegarty, and Direct Route, the group that won the Rathcormac/Fermoy bypass project on the Cork-Dublin road.
That group includes Irish construction heavyweights Sisk, Lagan and Limerick-based Roadbridge.
The motorway will serve the main route linking Dublin with the north-west but is expected to attract most of its traffic from Co Meath towns in the Dublin commuter belt, where the population has grown rapidly in recent years.
The M3 will replace the single-lane N3 between Clonee on the Meath/ Dublin border and the northern outskirts of Kells.
It will include bypasses of Navan and Dunshaughlin.
But construction work, which is scheduled to last around three years from the date the contract is signed, could be delayed by ongoing controversy surrounding the proposed route for the new motorway.
Environmental activists have threatened court action to block the project because it will include construction work near the Hill of Tara, an important archaeological site.
The new motorway will be the fifth toll road in the country.
Tolls are currently applied to vehicles travelling on Dublin's East Link and West Link bridges and on the M1 motorway linking Dublin with Dundalk, Co Louth. The Kilcock/Kinnegad and Rathcormac/Fermoy bypasses will also be tolled when they open for business.
Outline plans are in place for further toll roads, including a proposed bypass of Waterford.