A couple of days ago, I had blogged about Automating 3270 emulators with EHLLAPI. While looking for some more information on it, I came across the EHLLAPI Programming Reference published by Alebra Technologies.
Its almost a 90 pager PDF guide that explains well in detail about all the functions that can be performed with the EHLLAPI along with how you can go about implementing them. Even some gotchas that you need to look for.
There is also a very nice sample provided by Luca Bertoncini at Code Project titled - "Using EHLLAPI in C#".
Overall, these are very nice and helpful resources that can help you get started quickly for automating the 3270 emulators.