Marley Marl,
What follows is mostly theory that may not translate precisely to real life operations, but it might help you decide what to do.
1. All floating boats sink down into the water until the weight of the water displaced by the hull equals the weight of the boat. If a pontoon boat has its weight evenly distributed on two symmetrical pontoons, the depth of each pontoon will indicate the weight of the boat. The depth will also reflect the shape of the toon. A fatter, longer toon won't have to sink as deeply to displace enough water to support the boat.
2. Unless it has some lifting mechanism (i.e., strakes or flat bottom toons), it pushes its way through the water when underway. The more of the toons that is beneath the surface of the water, the more water is has to push, so the more power it takes to go at any given speed. A good indicator of how much power it will take is the cross sectional area of the part of the toons that are below the surface of the water.
3. If you add a third toon just like the other two you will add weight and more lifting volume. The added weight means you will need to displace more water total, but as the total weight is being supported by three toons rather than two, each will sit higher in the water than before. But as the engine is pushing more weight that is displacing more water, it will likely be slower.
4. The bottom halves of the new logs you are considering are effectively round, and I assume your boat will not be heavy enough to push them more than half way into the water most of the time. So we can compare then with your current logs as follows. A 21" log has a cross section that is about 10% larger than a 20" log. A 23' long log has about 4% less volume than an identical 24' log. So each of your new logs displaces roughly 5% more water than your current logs. If you replaced two for two, I doubt you'd notice the difference. Replacing two with three will cause your boat to sit slightly higher, but not likely run any faster.
Bottom line: I suggest you do as was suggested above. Focus on the engine/prop problem first. Then, if you want to go faster, get logs that will lift your boat when it is underway. Just changing from two logs to three likely won't help.
