I do not have experience with TN3270; therefore, the remainder of the post may or may not be helpful.
I suspect you will need to develop some type of custom DPH and possibly a custom delimiter to parse the request / response into a usable asset in the service.
I saw a similar type of mainframe connectivity tool called JNative client. JNative uses TCP and sends what amounts to a screen scraped command line with input to the mainframe. The requests on the wire contain a combination of EBCDIC and ASCII characters. The EBCDIC characters, which appear to be control information, wrapped the ASCII user input sent to the mainframe. I had to create custom logic to programmatically determine the request / response delimiters because JNative led the payload with several Hex record lengths, userId information in binary format, and padded a variable number of 0x00 characters to align each request and response to a full word boundary. (Probably because the mainframe was expecting data to align to a full word boundary)
During recording, I also found that the response payload generated in the VSI translated as binary due to the EBCDIC characters (0x80, 0x00, 0xFF, etc.) in the payload. I spent a considerable amount of time unpacking and repacking the requests / responses in order to make the VSI response editable. I had no documentation as to what the makeup of the request and responses looked like, so most of my parsing was done on a best guess hack basis. I was able to get a working prototype in place. Had we gone beyond a prototype we would need to re-engineer some of the work due to findings that were uncovered as we learned more.