For a better example of 3 small engines pushing equal weight vs. 2 larger engines, the Falcon 900EX and the Embraer Legacy 600 have max takeoff weights within 100lb of each other @ an average of 46,651lb.

The Legacy 600's AE3007-A1Es have 7953lb of thrust each, where the Falcon 900EX's TEF731-60s have 5000lb/each. That's 15,906lb of total thrust for the Legacy and 15,000lb thrust for the 900EX.

The Legacy's two engines cost 21.4% less per hour than the 900EX ($447.09 for the Legacy vs. $568.48 for the Falcon)...but over a 1000nm segment, the 900EX will burn 18.5% less fuel than the Legacy (4150lb for the Falcon vs. 5090lb for the Legacy).

The Falcon's 140 gallons in fuel savings over that 1000nm segment more than offsets the higher costs associated with its third engine.

Of course, that may not hold true when you consider a MD11 vs. a A330 or B777...