I do not have the data, but the new engines have some very high end geometry to their blades. A lot of that is possible due to where they are located; on the wind in the wind stream. There would be some efficency lost due to the partial disturbance of the airflow on a S-duct type of configuration of an enter-mount engine.
That said the new engines burn a lot less, require less maintenance and because of this using two works better than three. There large engines that they are using today have a ton of added thrust for take off and being able to perform the 2-4 stage climb gradients required. What it also means is that they are pulled fairly far back while in cruise. Their fuel flows are way below what you would see with a three or four jet engine.
Trust me if it were more efficient to have three, you would see airlines buying them. They are not. Add it cost to carry, maintenance, added burn etc, and there is no comparison. remember that airlines are taking pillows off of airplanes to save weight. EFB's save weight. Every added pound costs a ton of money over the operational span of a jet.