Does 3 inches stand as "too far"? Or just borderline "too far"?
There is no fixed limit, your body will experience successively worse tolerance the more you lengthen. The further your body is altered beyond its natural state the more prominent the effects of a change in biomechanics will become.
Depending on how much athletic ability you are willing to sacrifice 7,5CM may indeed be too much or perhaps it will be acceptable in the context of your expectations. However, you
will suffer a decrease in athletic ability -prepare yourself for that
fact.
EDIT- The closest one gets to a fixed limit is, from a medical standpoint, your tibia to femur ratio. Your tibiae can not be longer than your femur or your gait will suffer. Not sure what the max recommended deviation is on the other end though.