The new panel replaces an informal arrangement between Costain and a number of firms, including CMS Cameron McKenna, DLA Piper and Norton Rose.
Head of legal Tracey Wood said the company had chosen firms which shared its values, selecting three from the initial 12 applicants.
The panel will now handle the lion's share of Costain's legal business, with Slaughter and May remaining as corporate adviser.
The company will also continue to use Clyde & Co and Reading-based Blandy & Blandy on health and safety and employment issues.
"We want firms which have experience and are at the forefront of the sectors we work in," added Wood.
Costain is one of the UK's largest construction companies and recently finished work on the new international railway terminal at Kings Cross St Pancras. Its annual legal spend is around £1.5m.
The panel will deal with health and safety, employment and some corporate work.
Wood added that Costain wanted to apply supply chain management procedures to its legal business.
Pinsents head of construction Richard Laudy, Chris Jarman of Bevan Brittan and Nigel Proctor of Addleshaw Goddard will be Costain's relationship partners at their respective firms.
Pinsents has a particularly strong relationship with the company, which along with other contracted builders, received legal advice from the firm during the construction of the Kings Cross St Pancras terminal.