Instead of three different, but similar, genes each with its own regulatory sequence and set of transcription factors, we more efficiently package it all into one gene/one promoter that is capable of making three different proteins.
So how would this process facilitate the evolution of metazoan life? Consider the picture below:
What this is telling you is that different cell types can express different versions of the same gene. And this is precisely the strategy that is used by the various cell types of your body (See my blog, “Why are there no prokaryotic mice?). Let me give you a classic example below the fold.