> _declspec(dllexport) char* __stdcall Test1 (char *str1, char *str2,
> char *str3 ){
> char* s;
> s = (char*)malloc(sizeof(char) * 256);
> sprintf(s, "%d, %d, %d", strlen(str1), strlen(str2), strlen(str3));
> return s;
> }

...

> DECLARE EXTERNAL FUNCTION TEST1 CHAR(256), CHAR(256), CHAR(256)
> RETURNS CSTRING(256) FREE_IT ENTRY_POINT 'Test1' MODULE_NAME
> 'FireBirdUDFs.dll';

Input parameters should be CSTRING too. And use cdecl.

Ivan