Three blade props are less effecient, but not by much. The loss is from the tip and hub drag of the extra blade. Also more frontal area and drag for the same thrust. But they are also smaller and have less tip speed, even well below Mach I this helps to reduce the losses.

Gear reduced props are only used on the turboprop engines, modern piston engines do not use gear reduction, that was only the huge radial engine airliners of the past.

There are two major reasons to use multi blade props. The first and most common is for ground clearance with a larger engine. The second is for better high speed performance at altitude. For example a large prop will not do as well at high speeds and altitude because it turns a larger column at lower speeds than a smaller multi blade prop pushing a smaller column of air at higher speed. That is why on many GA aircraft you will find that the same plane made for high altitude and presurized cabin will have a turbocharger and three bladed prop, vs the two blade prop for the non pressurized version.