Which institution (government or private) provides the complete data on HS (Harmonized System Code), tariff schedules for India, China and US? Was looking out for options on building a data store having HS code and tariff schedule data for few countries, which I can use as a base for import calculations and general HS lookup.

Is there a government or private body (specifically for the countries US, China and India) from which I can source this data in a digital format, which of course I may further process according to my storage model.

Just a note: I'm aware that there already exist websites and applications which provide cost calculations services as limited free and on chargeable terms.
The best place for this information is WTO TAO. The data does require a little cleaning but you have information on a countries' mfn