I haven't used this, just DuckDucklGo'ed it, but what about
KS DB Merge Tools for SQLite is an easy to use diff & merge tool for SQLite databases. This tool allows to compare and import/export database objects and data between two database files. It can run in two modes - basic free version and paid Pro mode that provides extended functionality and ...
Your requirements, as stated, are minimal.
WinMerge will accomplish what you want. Specifically, it allows comparing three text files.
It is open-source, with the source code located here.
It is freeware and donations are accepted here.
kDIFF3 will, as its name suggests, allow you to diff 3 files.
It is free, open source, well supported and :
compares or merges two or three text input files or directories,
shows the differences line by line and character by character (!),
provides an automatic merge-facility and an integrated editor for - comfortable solving of merge-conflicts,
Meld allows three-way comparison of files.
It is free and open source. There is an installer for Windows. Not that you need it, but others stumbling on this question might be interested in other platforms: It can be installed using the package manager on common Linux distributions; and though Mac isn't officially supported, there are some installers ...
If this is for professional development, then your version control system should diff the files for you. If you have no version control, then shame on you ;-)
In any case, if the websites were remote, you could use you could use HTTrack to download them.
Since you say that you made changes, then I presume that you already have them locally. In which case,...
I'm looking for an open source tool to compare two C++ sources and tell me if they are the same after disregarding comments, formatting and identifier renames.
That could be a research project, or at least a work for a Master's degree dissertation (and probably gets you a PhD). I guess that your ill-defined problem falls into Rice's theorem so is ...