I looked into this, and this is my limited understanding.
I use all the cassette when using the middle ring,
Top three gears only when using the big ring and only the lowest three gears when using the granny.
Using a long cage, gearchanges are not as quick and precise as they would be with the med but if I did through lack of concentration or accidental shifting find myself using all the cassette in all of the rings it would not be ideal, noisier and putting more wear on the drive train, but over a short period of time no real damage done.
With a med cage and 3 rings if i shifted correctly using only the ideal ring cassette combos you should be ok but if you accidentally found yourself in the granny ring/high gear combos for instance the deraileur would not have the lengh to take the required amount of slack out of the chain and it could drop off.
I think.