My CO2 line comes out at a fixed output pressure (15 PSI at the moment) and then I have a two-output manifold (both are at the same pressure--was too cheap to buy the ones that have individual pressure outputs). I used the Y-splitter in the past to carbonate a 3rd keg so it would be ready to swap as soon as one of my two serving kegs ran empty (already cold, too). Now, in my newly-designed wet bar, I only have room for the two kegs, so I probably won't use the splitter for anything.
In answer to your question, I think the only reason to buy a 3-way manifold is if you want to be able to (A) turn-off the third connection or (B) use a different pressure for the three different connections.