This could become quite lenghtly if I wanted it to be LOL.
to sum it up I agree with what everyone else said.
I had 3 dogs for years (until me & my ex broke up & yes I miss the dog more than him lol) but besides one addition to the major things like expenses (ex. food vet supplies), more potty time, care, grooming, cleaning & dividing attention. The excersize was not much different as I would excersize all 3 at once.
Like mentioned before yeah sometimes (it has to happen at least once) they may have their little (or big for some) tufts. Or one may bug the other (therefore cause problems like mentioned before for ex. not drinking etc.) but as time goes on you learn their individual personalities & kinda know how to prevent them or solve them or who to blame.
Me myself personally overall would reccomend a third dog to someone as long as they had the appropiate time, money & understanding. The only thing that is holding me back from another canine is finances.
I personally also would wait until a dog is about a year of age before adding another just because by then your other pup is out of the puppy stages (or just new dog stage if rescued as an adult) and you know them better by then, you know approx. if you can afford more $ & time by then and lots of other reasons but overall it gives you more time w/ the new addition at first.
I do reccomend having your current dogs meet the new addition first just to get an approx. idea as to how they will get along, even if there is a tiny scuffle they may be able to live together as if you are a good owner it only gets better with time!
If you can handle the way more extras for a puppy then that is USSUALLY a better chance that they will alll live together happily. BUT MANY many many shelter/foster dogs need good homes & you may get lucky & find one that has a background therefore you know that they are potty trained & how socialized they are etc... And even if not most shelters & rescue organizations will grant you a trial period, if things do not work out you could bring the dog back & try again with another.