My suggestion is to NEVER introduce strange dogs inside the home territory of one of them. Only two of the most amiable, boundary-lacking dogs in the world wouldn’t mind this. Neither of the breeds you are dealing with are those dogs.
You will need two people. Introduce strange dogs one at a time (do not gang up on a single dog), on leash, in a neutral (i.e. not the territory of either dog) environment in which you are going someplace. You should all take a long walk together without asking the two dogs to do anything other than walk properly on leash. Do not “introduce” them (leashes make dogs tense).
If this goes well, release the two dogs in a fenced, neutral area. Watch them every single second and stand close to them. Do not engage in conversation, stay totally focused. Educate yourself about dog body language – most people know zero about it. Watch for dominance postures and gestures, anxiety postures and gestures. Intervene BEFORE things escalate, which can be unbelievably rapid, but almost never without warning signs.
To the extent possible, let the dogs work things out with each other without trying to direct anything. Do not talk.
If this goes well, do the same thing with the other one of the pair of Dobes.
Next you can walk the GSD and one of Dobes by your house. Again, no talking, watch the dogs closely. Then around the back and front yards.
If it was me, even if all seems cool, I would never ever let all three dogs loose together even supervised, and I would keep them safely separated inside the house. Inside the house is not just territory, it is a den, a lair, to a dog, and permitting a strange dog to wander about it is too much to ask the majority of dogs, much less dogs actually bred to guard a territory like Dobes.
I own a guardian breed, and like everyone else who works with these in a serious way, I wouldn’t dream of bringing a strange dog home to my pack unless it was a puppy (adult dogs will not hurt puppies no matter how annoying they are, generally speaking). If I had to care for a strange dog, it would be crates and kennels for all. Just too dangerous.
Also watch for marking. Your perfectly housebroken Dobes may feel the need to lift their legs on the furniture with a strange male’s scent floating around there.