If you already have two, a third of about the same size SHOULDN'T be much more work...BUT, before you commit, try bringing the other dog over for a day or two to see how they all get along.
You can try that, but most times one or both of the established dogs won't interact, or interact "nicely", with the new puppy, like OCGuy mentioned with his dogs.
And it's all about the established pack and an intruder. Have seen behavior ranging from ignoring the puppy to sort of nipping at it...but it's all to put the pup into its place relating to pack standing/hierarchy. But since you---the owner and alpha--show the others the pup is going to be coming into the home, the dog alpha will grudgingly accept the pup in a few days, followed by the other dog. Sometimes the beta dog will actually accept the pup faster than the alpha, but doing that may incur the wrath of the alpha....group hierarchy/pecking order must be maintained.
Then, a week or two go by and you find the dominant dog playing with the puppy. Rarely goes much different, although in some cases, the dominant dog never accepts the pup....rarely.
We have 4 dogs, ages 10, 9, 6, and 4. The two older dogs were introduced individually as puppies into an already established household with two older dogs. Fit right in after their respective "break-in periods". The two younger dogs were sorta replacements for the original pair as they passed. Again, both fit in well.
The biggest thing is let the dogs sort it all out. You really cannot force it to move faster. They'll probably act like they hate the puppy and hate you...for being a traitor and bringing in that "thing".
But it is fun.