There's a big variable called weight. Without that there's no answer that's going to do you much good.
To illustrate, if you have a pair of old, say 18"- 20", pontoons that are mostly submerged when just sitting there, and you add a 25" or 28" pontoon between them, clearly the boat will float higher.
On a very lightly loaded newer pontoon with a pair of 25" pontoons, that boat is going to be mostly out of the water already. Adding a 3rd 'toon isn't going to have near the same effect....
As far as speed, without more power, that 3rd toon is going to create more drag. Unless we're talking a lot more power, and a 3rd toon with lifting surfaces, I doubt your boat is going to go any faster.
