The structural argument
One agency with three departments is easier to explain and worse to buy from. Departments compete for the same margin, the same senior people and the same client attention, and the client cannot tell which part of the invoice bought which result.
Three companies with their own P&Ls cannot hide inside each other. Each one has to be the best answer to its own market or it loses to a specialist, and we find that out from the market rather than from an internal review.
What it costs us
Duplication, honestly. Three brands, three positioning problems, three sets of client relationships to maintain. A single agency would spend less on marketing itself.
And a harder sales conversation. Buyers who want one throat to choke sometimes read three names as three vendors, which is why the group site exists at all.
What it buys
Three uncorrelated demand cycles, one shared delivery bench, and the ability to sell a client the one thing they need without the pressure to attach the other two.