The CiU faces little competition for Catalan right votes from the PP, but the ERC faces a more serious threat from the PSC.  In the event of an CiU-ERC coaliton, if the ERC is able to marginalize PSC by forcing Madrid to rein in the PSC affiliate, the may be able to shake free another 10-15% from the PSC vote total.  That would create a distinct, divergent nationalist, and left right cleavage.

Conceptualizing the vertical axis as the nationalist cleavage, and the horizontal as the left right you get the following:

                     Nationalist                    
                     ERC  |  CiU
               Left ==|== Right
                     PSC  |  PP                    
                   Anti-Nationalist    

And I'll give my consent to any government that does not deny a man a living wage-Billy Bragg

In fact, the 3-party coalition was described as progressive catalanist with emphasis on the catalanist.

Note also that the Left swept the National Senate elections in Catalonia last time (12 senators to 4 for CiU) but the way this was achieved was that the PSC did not run under the PSC/PSOE banner but under Entesa Catalana de Progrès [Catalan Progressive Entente, see this old comment of mine], in other words, the 3-party coalition under a progressive catalanist banner.

Those whom the Gods wish to destroy They first make mad. -- Euripides