[gemy]

I am with others here; in the fact that if there was a quick change from active, energetic, play, exercise needing and enjoying dogs, to quiet, no interest in walks, toys, or games you played before the neutering; then I think more investigation is needed.
There might be some other health issue going on, your vet is the best one to speak to about this.
While at present none of my dogs are neutered, as they have "aged" , they have matured in their head, but are still very interested in life. In our walks, in our swims, in playing games, etc. In the past, our neutered animals did not display indifference to their usual "prespay" activities. But yes there was a tendency to gain weight. Amount of food versus exercise, needs to be considered, and adjusted accordingly, to acheive optimal weight and health.

Also this might be the time to sit down and evaluate your interactions with the two older dogs. I'm not too sure, but it seems like you recently got another dog/puppy into the house, ie your Yorkie. Are you spending less time with your older two? Doing things differently; like have the sleeping arrangements changed? Working more/less? Different house? Do you make the time to have one on one time with your older two? Has their food changed? Just sit and think about things as objectively as you can.

In terms of neutering your Yorkie, try to have an honest discussion with your vet about the pros and cons.