Without regarding the veracity of the drawing itself, is there anything in the concept of a 3 engine bomber that would make it less efficient than a 2 engine or 4 engine bomber? Usually, one adds more engines if the available engine isn't powerful enough for the requirements.
With single engine, it has to go in the middle. usually leading to bomb bays on the side of it. With 2/4 engines, the bomb bay can be in the middle. With three engines, one can have them all together, for widely separated bomb bays, or if that's not very good, one can separate the engines a little and put bomb bays between them. With separation distances not really differing from what B2 has.
On the other hand, the less engines there are, the more fuel efficient the system is. And operational costs may also be a tad lower.
Basically, if requirement is such that a very large bomber isn't needed (say intercontinental range at 10 tons of weapons not needed) and if available engines are such that 3 of them would be a perfect fit - would it be so unusual to see a three engine design?