I've tried defining EXTJSON_SBJSON by setting my project's Other C Flags:
-DEXTJSON_SBJSON
It doesn't work: TTURLJSONResponse still uses YAJL.
I've tried defining EXTJSON_SBJSON by setting my project's Other C Flags:
-DEXTJSON_SBJSON
It doesn't work: TTURLJSONResponse still uses YAJL.
I'm not sure that the Three20 library is abstracted that far. Changing a flag isn't going to change the code that's written to use the YAJL parser. The SBJSON and YAJL parsers are completely different in terms of use. You'll have to rewrite the parts of Three20 that use YAJL to use SBJSON instead.
Both are supported in Three20, what you need to do is select the right one under the right panel when you add the reference to your project. Then go to your target and set the proper dependencies.