Maximum trijet mass
 

The heaviest twinjet is, I think, Boeing 777, with up to about 350 tons MTOW.

Probably because it is not possible for one engine to propel a bigger plane. Boeing 747 and Airbus 380 are quadjets and Mriya is a sexjet.

Can a big engine be mounted in the fin? And could a Boeing 747 or Airbus 380 derivative be built with three engines?

DC10, B727, L1011 Tristar have a engine on the tail-fin.
The L1011 max weight I think is 211 tons. Engines are wing mounted these days as maintenance is cheaper and easier.

Airliners wound't use only one engine due to ETOPS, safety, performance and reliability issues.

Engines
 

And that of MD-11 is 285 tons.


Sure. That is why 2 is the minimum. But a plane with 1 engine out of 2 inoperative has 1/2 the total installed thrust, whereas a plane with 1 out of 3 inoperative has 2/3 of total and a plane with 1 out of 4 inoperative has 3/4 of total.

Isn´t the maximum size of twinjet defined by what one engine can climb out with if the other is inoperative? Rather than by the available total thrust of 2 engines?

"DC10, B727, L1011 Tristar have a engine on the tail-fin."

Actually the DC-10 and MD-11 have an engine mounted in the tail fin.

The TriStar and 727 have the engine in the rear fuselage with the intake in front of the fin with an S-duct down to the engine.

Never a truer sentance written...

Sorry for the drift.