Is there a C++ compiler that I can use to go cross-platform with development?
I need to develop the same DLLs for AS400, Sun, and Windows.
I'm not sure I understand what you're trying to do. A dll is something you only have on Windows, and even on Windows you can bump into incompatibility problems such 16 bit vs. 32 bit versions. So a dll is probably out of the question. The other two options are:
1) Creating a single executable that runs on all three machines.
2) Writing code that is portable enough so that you can compile it on all three platforms.
Creating a single executable that can run on these three platforms is impossible. Each platform has a different CPU architecture that uses different assembly directives. On the other hand, if you intended to have a portable application at the *source code level* and then compile and link it on every machine separately, you won't need a special compiler. Simply make sure that your code is portable enough: avoid non-standard or platform-specific features.
The Code Warrior compiler is available on various platforms. If you wish to use the same compiler on every platform, you can check it and see whether it's available on AS400 and Sun too.