The 2-in-1 3-in-1 projects:
Your own code + looks for other people's code (not from this site), by signature checking.
And the code to put them together.
For mods of other people's work:
You could automate the steps too, except not the downloading (because RHDN download links are behind registration wall):
patch orig, delta1, tmp1
patch tmp1, delta2, tmp2
patch tmp2, delta3, final
delete tmp1, tmp2
Is it that hard?
If it is still too hard, someone could make a utility that does this pseudocode with an arbitrary number of links in the chain.