Although, I don't have more than 2 yorkies, I do have a poodle and Papillon that were foster kids, we chose to add to our family. Going from one to two was more difficult for me, than going from 2 to 3 and then 3 to 4. I had Ruby and Reno for 1 1/2 years together before Louis and Scout came along. Ruby and Reno stick together more and then Louis and Scout are inseparable. I do grooming on different days, as I also currently have 2 foster girls....too many for one days grooming
I think if you are confident you can take care of, and have the resources to own 3 yorkies then go for it......I do believe it will help Billie and Ralphie, as the new pup and Ralphie will tend to keep each other occupied.