Well, firstly, welcome to the Community modding section. Here, you and others can contribute to the wiki; thus information may not be as heavily detailed and sophisticated as Frictional Games' official details; but as members, we'll do our best to provide precise and detailed steps for you.
This is the Getting Started page, which will contain various links to places which should be of great advantage to you to get introduced into HPL3. Of course, whatever your qualities are, you may be able to get instructions specifically on those since this will be a generic introduction; but once you learn something, ADD IT TO THE WIKI! We need more information, and anything you can provide is beneficial!
Personally, I believe the best way to learn is to see how others do things. In this case, doing some research, understanding how to structure code, exploring levels and determining the relationships between the editors and third party applications will be of great benefits to you. '
Here are some of the things you'll need to get started:
These two mods which were officially released by Frictional Games should also be handy.
And these official pages from Frictional Games within the HPL3 wiki should also be of good use:
Other Links: