With regards what buses are there if there are 3 it is most and likely MS CAN-usually body information, HS CAN which would be powertrain information, and possibly and Multimedia CAN. Reading the buses is one thing but to change any data alot of the info tends to be write protected, it can vary for each diagnostic identifier what read/write permissions it has.
To identify which bit/s in what byte/s are for a particular function is painstaking work as a lot of the data is scaled. Alot of the engine data will be fixed periodic but the event periodic data should be easier (not easy) to spot.