What you want to do is no even near to an easy thing to do :)
You may need to write lot of lines of code and you need to understand digital wave format completely. It is no like a function to add an effect of merge different files and so on. MP3 is nothing except a compressor for WAVE data. Your main mission starts with WAVE after decompressing of MP3.
But any way,I strongly recommend using of a Library. It can help you a lot and all of them have active developers and forums for support and help. For example personally I recommend "BASS Audio Library".
This library have different plugins like "BASSmix" and "BASS FX" that can approach some of functionalists you expect.