The real question is, what do you consider to be efficient? There is a big factor of subjectivity in this problem (and a bit of nonsense in this thread, LOL)

At first it sounds simple: the goal is to get the most thrust for the least amount of fuel burn. The most efficient prop would seemingly be very big and spin very slow. But maybe this thrust monster doesn't move air fast enough for your airplane's wing to generate lift. It's not because the prop is inefficient.

So now the goal becomes: to get the most amount of thrust, at appropriate speed, for the least amount of fuel burn. So now the prop is a little smaller and spins a little faster. On a calm, comfortable day, flying a long straight path at constant altitude, the fuel burn is incredibly low. The flight back is on a hot, muggy day with choppy winds at altitude, and requires some travel over a mountainous region. The wind gusts change the load on the prop and the engine seems to need more fuel to keep RPM high enough for comfortable speeds. It is difficult to gain altitude to clear the mountains. A smaller prop with variable pitch would seemingly allow better management of rpm and air speed.

Now the goal is: most thrust, at appropriate speed, reduced size, variable pitch, for the least amount of fuel burn. So the most 'efficient' prop becomes something very different from: which has more thrust for a given amount of fuel burn, or which burns less fuel for a given amount of thrust.

I can't say I've read every post here, but I would bet there are many reasons behind the number of blades on any airplane's propeller that aren't mentioned. But it all boils down to: what does the customer want? If everyone's main focus was fuel burn, we all would be flying in hot air balloons. The choice of size, pitch, and number of blades on a full-scale airplane comes down to the purpose of the aircraft (how high, how fast, how heavy, how comfortable, and in what conditions), the expectations of the customer, and a million other factors related to the power plant, airframe, and the hundreds of other systems involved in making airplanes fly (fuel temperature/limits, fuel metering, oil temperature/limits, exhaust gas temperature/material capability, air conditioning/electrical system demand, etc, etc, etc.)

It is basically the same for our models. Our engines operate at certain RPM, have certain power bands, and can handle certain amounts of heat. Some of us fly fast, some slow, some hover and some glide around. Some airplanes are slippery, some are dirty. Some take-off on grass, some on paved runways, some on water, and some are hand launched.

So given 2 propellers, one 2-blade and one 3-blade, of equal diameter/pitch/material, allowed to spin slowly in ideal steady-state (constant speed) conditions, and focused solely on which produces more thrust/energy input, my money is on the 3-blade.