Why is my Mango Pickle foaming with Gas after 3 days, it is beginning to overflow, even though I left 2" from the cap
This "pickle" has no acid whatsoever, which is needed to pickle preserve the mangoes.   

I can only infer that it is intended to be a fermented pickle (perhaps akin to saurkraut, given the high level of salt), where bacteria break down the sugars in the mangoes and produce acid (typically lactic and acetic), and carbon dioxide.   The off gassing of the carbon dioxide can be seen as bubbling as the mangoes ferment, and would be a normal part of the process. 

The fermentation process would normally take several days to a couple of weeks (depending on temperature, mostly, and the size of the mangoes).   Is that what your recipe suggests?

If this is not meant to be a fermented pickle, then you are seeing signs of very aggressive spoilage.