A full Driver combo is when you go break > topple > launch > smash. Done correctly it does massive damage on its own.

A full Blade combo is a bit more complex, you basically need to sequence 3 blade arts ("a button" for yourself or "zl" or "zr" for your other party members) in a way that the blade art tree allows you to (the thing on the top right corner of the screen).

I didn't know this was a technical term, but I'm assuming a fusion combo is when you combine a driver combo with a blade combo (there also is usually an announcer that says something like "Volcano, topple" or "Sandstorm, launch"). This one does massive damage especially if you manage to get the third stage of a blade combo on the launch stage of the driver combo (you could maybe get it on the smash stage but I've yet to pull this off since smash lasts like literally one second). Fusion combos also give other effects to the blade combo, such as a longer amount of time before it expires.