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 ...