Need tool that allows for manual merging of 3 files creating a new 4th file

As a developer, when there is source contention on a file, a tool is needed to help resolve that contention by generating a new file containing the differences manually selected by the developer.

A tool is needed that will assist in resolving contention by performing merges of 3 source files and creating a new 4th file as output. This tool needs to let the user easily view and manually select each difference that will be applied to the output file.

We need this tool mainly for applying new releases of IBM's zTPF code to our zTPF systems, but would use it at other times also. This tool will be an integral part of our zTPF release upgrade process.
When applying a new IBM release, when IBM has updated a segment in that new release that we also have user modifications in, this tool will allow us to merge an 1) current release IBM Vanilla file with a 2) new release IBM Vanilla file and a 3) user modified file, allowing the user to view and manually select each difference, to generate a new 4th file that will contain the user modifications and the new IBM release updates.

The IBM 3-Way Merge Tool that was in the TPF Toolkit 4.2 served this purpose for us, but since it is being dropped in TPF Toolkit 4.6, we need another solution. As mentioned above, this is an integral part of our zTPF release upgrades.