So...what makes you think your 3 year old Labrador actually NEEDS a `companion` to play with.
Whats wrong with her owners being her companions?
A dog that is walked adequately daily and given plenty of mental stimulation does NOT "need" another dog around.
Its not a case of will a German Shepherd dog get along with the Labrador, but more of will the Labrador accept the GSD puppy.
However...Its very likely though that an adult dog WILL accept a puppy and get along with it, but there is no guarantee that as the pup matures they will still get along, especially if both want to be top dog.
A GSD is an entirely different kettle of fish than a Labrador and has different inborn traits.
Loving the breed is fine, but you also need to know how to handle and train it and that is not quite as easy as training a Labrador. GSD`s can be very willful and need lots of mental stimulation and exercise.
If you go ahead with getting a GSD pup I recommend you ask the breeder for a pup with a submissive or laid back temperament as I doubt you would be able to handle a dominant or very willful GSD if you have never had any experience with the breed.
Also, although compatibility of temperaments is paramount to two dogs getting a long, the odds would be best if you got a male. But your female should be spayed if not done already.
GSD owner for 56 years.