I know what you mean...I'm going through something similar. Scottie will be 2 years old in May and we recently got a 5 month old girl yorkie, Casey.
When Scottie was a puppy, he played really really well with his yorkie brother - even after leaving the breeders home and then revisiting his brother a few times later on. Scottie also made friends and love playing with a neighborhood Biewer yorkie mutt, who they met as puppies when they were both a couple months apart. Aside from these two friends, Scottie is fearful and timid of most other dogs. I wanted him to have a friend. I read that it's best that a second dog is introduced 1 to 2 years apart. So a year and some later, along comes Casey.
For the first week, Scottie was aloof and depressed. The first day, he wouldn't let me touch him after Casey licked me all over my hands. Each time she approached him, he'd low-growl at her to tell her to keep her distance. She didn't care lol...she's just a ball of bursting energy. So Scottie had to constantly dodge her.
It's about two weeks now and he seems to have finally gotten used to seeing her around and is actually instigating play with her when he's bored. I think he still finds her annoying because he's maturing and lost his puppy energy. She's quite possessive and is stealing all his stuff (beds, toys, food, water). He's super passive and is letting her steal all her stuff and still dodging her (drops the toy he's playing with as soon as she comes charging towards him). Heck, he doesn't get to play like he did before anymore since she's been around (we used to play fetch at home all the time).
I feel bad for Scottie. He's either a gentleman or just a big wuss. It's a little weird parenting them as I've never been an actual parent before. But very slowly, they're bonding and becoming friends. I really enjoy seeing them play. Last night, while I was washing my dishes, they chased each other the entire time. By the time it was bedtime, they were pooped and both slept soundly through the night!
Good luck with your possible third. It's quite stressful! But hopefully all worth it with time
