In just 2-3 years time 3 engined planes will be very very rare birds as passenger carriers. MD-10s and MD-11s will carry on as cargo planes into an unknown future.
To know why they disappear you must realise why they were built. They were built in a time when twins were not allowed to fly any substantial distances over water. Today ETOPS twins cross all oceans.
No really new tri-jet has been designed during the last 30 years, and it will never happen again. The blended wing-fuselage design will not happen as commercial transport because such a design will have too narrow balance limits for practical use. The B-2 bomber plane will be the only example, until this planet comes to sense and bomber planes goes completely out of fashion.
Best regards, Preben Norholm
Always keep your number of landings equal to your number of take-offs