DR
Indeed I was as well , I was prompted with the three engine look by memories of some Boeing ideas for the 75 before they actually built it . I think as someone said the 757 was close to being a replacement, certainly wonderful field performance but lacking in range . So realistically one would be looking ata VC10 with CFM 56s but they didnt come along for many years to help Douglas upgrade the venerable DC8.. The Trident on the other hand probably could have been built with bigger semi /low bp fan engines as originally intended and would have been much closer to the 727.
But it seems in the airline industry there is no point in building something thats not quite right , better to husband resources and try and skip a generation