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.
If you aren't in the market for a new regulator, you could either get a Y which is the cheapest option, or get a 3 way manifold. I personally wouldn't run 2 manifolds unless it's somehow cheaper for you...
How you split the feeds only matters if the gas is flowing through at a high rate. Simply pouring a beer isn't going to make the gas flow enough to make a difference. The pressure will still equalize across the entire system faster than you can deplete it from any exit point. Even in a party situation where there's a beer poured every minute, or every ten seconds, there's PLENTY of time for the gas pressure to equalize through the entire system.
The real benefit to a manifold versus a simple splitter would be the shutoff valves that manifolds usually have. Without the valves a manifold is nothing but a more attractively packaged splitter. If you don't need to turn each gas line on and off independently, then just use the splitter that NewBrewB posted the pic of. All kegs will be under the same pressure. (There can be other benefits to a manifold vs a splitter, but those benefits will only really come into play at much higher flow rates than dispensing beer.)
Disclaimer: I'm a complete newb with kegging, but I do know a bit about fluid dynamics.