Cobbetts, DLA Piper and Weightmans have won places on the North West's first-ever multi-borough panel.
The Gershon report on public sector efficiency called for bodies to combine services and the first boroughs in the region to respond - Halton, Knowsley and Sefton metropolitan borough councils and West Lancashire District Council - have pooled their legal spend to boost economies of scale.
West Lancashire District Council secretary and solicitor Gill Rowe said: "This review has been advantageous for all four boroughs, but particularly for us, because it gives us the scale to be able to use firms such as these, which we wouldn't otherwise have been able to afford."
Cobbetts, DLA Piper and Weightmans have been chosen as the panel's 'main partners', meaning that they will work with the councils on initiatives such as joint training and shared library facilities.
Other firms selected include North West firms Brabners Chaffe Street and Mace & Jones for employment work, Brabners and North West firm Stephensons for civil litigation and prosecutions, Addleshaw Goddard and Warrington-based Forshaws Solicitors for commercial and general conveyancing, and Stephensons for social services and education work.
Thirty two firms tendered for panel places, with 13 being chosen for interview at a two-day event at Aintree Racecourse last September.