Are you serious about meaning Informix 3.30? As in, 3.30.14? As in, a product that was last released in 1986? It creeps out of the woodwork every so often, but hasn't done so for quite a few years now. This is the pre-SQL product, with the database dictionary (catalog) in a file 'database.dbd' and the data in files such as 'table.dat' and the indexes in files such as 'table.idx'?
If so, then there are extremely unlikely to be any ODBC or JDBC drivers to connect to it.
However, you should contact me directly - see my profile page - and we can discuss what options you have. Suffice to say I still have access to the source code, and I know how to get data out of the system. I'll need to know a lot more about your environment and whether you have any of the Informix 3.30 software still running, etc. (I have Informix 3.30 running on MacOS X 10.6.2, compiled in 64-bit mode, if that reassures you at all. But I thought it was just a joke.)
If you don't have files such as the '.dbd' and so on, then you (probably) don't have Informix 3.30. We'll also need to discuss what you do have.
I presume you've found Simba and their discussion of their DSI (Data Store Interface) technology. Is the 3.30 database still in active use, or is this a one-time data transfer? Your options will be somewhat different depending on which applies.
If it is a one-time transfer, then we can do the work with a tool kit I assembled over the period 1986-1996 (approximately); the result will be ASCII files in a format that can be loaded into the DBMS of your choice.
If it is an ongoing operation - the Informix 3.30 database is still in production use despite being somewhat beyond the end of its normal life - then you should be looking at the services Simba provides, and a custom DSII (DSI Implementation).
Informix 3.30 comes with the A.L.L (Application Language Library) that you can use to access the data from C (and hence C++) code. You'd be using that to get to the data in the DSII module that you implement - or find someone else has already implemented.