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.