(I am the debian maintainer for tiff and tiff3)
A transition is nearing completion in debian for tiff, and once it is done, I will be requesting removal of the tiff3 package. However, I don't expect this to happen before the sync freeze for trusty. I don't know if you have a way to do it, but it seems like excluding tiff3, which no longer supplies any dev packages, from trusty might be a good idea as it will reduce by two years the amount of time people have to worry about backporting security fixes. The tiff packages get lots of security bugs filed against them, and it's getting harder to backport these to the 3.x series. At this point, assuming everything successfully builds from source for trusty, there should be no binary packages in trusty that depend on tiff3.
It might be that there are no original Ubuntu trusty packages with dependencies on tiff3, but there are third party packages.
As an example some of the canon printer/scanner driver packages depend on libtiff4. With the removal of tiff3 from trusty that dependency now is no more satisfiable and the scanner drivers can no more be installed.
e.g. cnijfilter-
Package: cnijfilter-
Version: 3.40-1
Section: graphics
Priority: optional
Architecture: i386
Depends: libatk1.0-0 (>= 1.9.0), libc6 (>= 2.3.4-1), libcairo2 (>= 1.0.2-2), libcupsys2 (>= 1.2.1) | libcups2, libfontconfig1 (>= 2.3.0), libglib2.0-0 (>= 2.10.0), libgtk2.0-0 (>= 2.8.0), libpango1.0-0 (>= 1.12.3), libpng12-0 (>= 1.2.8rel), libpopt0 (>= 1.7), libtiff4, libx11-6, libxcursor1 (>> 1.1.2), libxext6, libxfixes3, libxi6, libxinerama1, libxml2 (>= 2.6.24), libxrandr2, libxrender1, cnijfilter-common (>= 3.40)
How to proceed with these? Any possibility to add "provides libtiff4" to the lilbtiff5 package or anything else?