AS/400 had a terminal that was similar to but not compatible with the 3270 used on the 360 mainframes. Applications like either one are a lot like web applications from 1999, that is, the mainframe draws a screen with a form in it, the user fills out the form, hits a button, and it get submitted.

The programming model for transaction managers like CICS on the 360 was similar in some ways to back end web frameworks, they even had clever code generation systems (see “X macros”) for writing serialization/deserialization in assembly language.

AS/400 terminal is 5250. Like you said, it's similar to 3270 but different.

AS/400 (and contemporary IBM i) actually supports 3270 as well - although I don’t think many people ever used that support