i heard this rule of thumb many times and just don't get it as long as your 1 year old is properly trained and socialized/loves other dogs. just wanted to share my experience and hear about other experiences on the subject.
i totally disagree with the 3 year rule. as long as your first dog is highly trained and loves to be around other dogs.
we got our first dog named julie at almost 5 months old. never socialized with dogs or people, was extremely terrified to even walk past a person, she would crouch down and stop walking if another person walked by during walks, never house trained, didn't know a single command.
we put her into basic classes which my wife did 'cause they are done at her job. trainer does all types of training but his main thing is his company trains and sells dogs to the military and law enforcement. she was a star and picked up everything faster than normal. we started late in her life and before she was a year old she got her CGC and was in advanced classes with the mals that have been doing it for years and giving them a run for their money. all off leash obedience/rally, agility and poked our noses into explosives detection.
all the time julie was going with my wife to her doggy day camp job everyday. spending 8 hours a day playing with dogs of every breed and training level. she really loves playing with dogs and it's great for her dog socialization and all the exercise she gets. she turned out to be so good with dogs that she is used as the interview/meet and greet dog to see if new clients' dogs are compatible with other dog before their dogs can attend camp.
on my wife's days off or when wife's work was too busy and she leaves julie home, julie just wasn't the same. a spark was missing i could say. wife suggested another dog, i said **** no! 2 cats and a dog is enough. wife talked me into it. when julie was a year and 4 months we picked up another GSD, 8 weeks old. what do you know, they were immediately best friends. completely inseparable and best yet julie is just overall a much happier dog with another dog companion 24/7. we can so easily how much a happier all the time julie now is. not that she wan't happy before but she is overly happy, smiling all the time. when you look at your dog and can tell they are happy it makes you happy. i'm more happy to see julie happy ALL the time now.
the best decision i ever made was getting a new puppy for my 1 year old
5 minutes together
2 weeks together
2 months together