Plenty of objections here.
A solution could be a cross section offering 777 like maindeck flexibility: 9-10 abreast and a full length upperdeck significantly smaller then the A380, but far more practical then the limited 747 upperdeck.
But this is problematic. If you look at the Ecoliner cross-section and compare it with 747 cross-section, it is glaringly inefficient, structurally, and would be extremely heavy!
You have very thin sidewalls, and absolutely no double-bubble discontinuity. This means that your thin sidewalls have to carry immense bending loads, and therefore be very heavy.
However, letīs imagine a full-length doubledecker with the exact cross-section of 747. You could shrink or stretch it, too. The 747 with short double deck was successfully stretched to 56,3 m of 747SP. Now imagine a 747SP with full length upper deck. What would the passenger capacity be?
And you can stretch it to 747-800 length, keeping the full length upper deck. This should cover a range of passenger capacities...
A compromise could be to fit such an aircraft with 2 large engines and an APTU (Auxiliary Power and Thrust Unit) providing thrust during critical flight stages. Boeing studied it during the nineties. A twin + APTU seems to offer significant saving over a conventional four engined configuration.
But what about a genuine three-holer?
Think of MD-XX. DC-10 cross-section is only slightly narrower than that of 777 - DC-10 is 602 cm wide, 777 is 620 cm wide. They have the same maindeck flexibility: you can seat 10 abreast on 777, though 9 abreast is more comfortable, and you can seat 10 abreast on DC-10 or MD-11 - Finnair does, although 9 abreast is more comfortable.
If you compare MD-11 with 777-200ER, they have pretty similar capacities: cross-section of 9...10 abreast as described, length 63,7 m for 777-200, 61,7 m for MD-11; maximum of IIRC 440 seats on 777-200, 410 seats on MD-11... MD actually fitted 421 seats for the evacuation test, but one passenger broke her neck, so only 410 seats are allowed.
Now, look at the MD-XX family, and compare it with B777-300ER/B777-200LR and A340-600/A340-500 families.
Total wingspans are pretty similar: 64,8 m for all three families, specifically to fit 65 m box.
The MTOW-s are 350 tons for B777-300ER/B777-200LR family, 380 tons for A340-600/A340-500 family, and 365 tons for MD-XX.
As for total length, it is 63,7 m for B777-200LR, 73,9 m for B777-300ER, and the MD-XX family has 61,7 m for MD-XX LR and 71,2 m for MD-XX Stretch.
One notable difference is wing area. A340-600/500 have somewhat more than 420 square m, B777-200LR/300ER something similar (how much exactly?), whereas MD-XX has 483 square metres.
How do you think would MD-XX Stretch handle?
And what about a three-holer from a generation after MD-XX?
Mind you, a humped partial doubledecker like 747 cannot be a trijet - Boeing tried to build one, but could not because the hump disturbed the airflow into the tail engine. But there is no reason why a full-length doubledecker cannot be a trijet, any more than there is a reason why a full-length widebody singledecker cannot. So, if a DC-10 stretched once more past the MD-XX Stretch does not give enough capacity, you could consider a doubledecker trijet.