Cyclic Hotline,. .There is one more penalty of the third engine that exacts a penalty for each mile flown - specific fuel consumption.
Unlike piston engines, turbines lose efficiency at part power. Each horsepower can consume much more fuel at 50% power than at 75%, about 20% more for the typical turbine. If 3 engines provide cruise power instead of two, the extra fuel needed for a given range causes more cost for the mission, and also reduces the payload that might have been carried.
This is a fundamental for turbines, not often appreciated by us pilots, who view more power as a solid virtue.