Hi and welcome to YT! Congrats on your baby.
You truly to have to watch out for hypoglycemia at this age. They can go down super fast when they are this small, too. You say you have Nutri-stat? I'm not familiar with that. Is it like Nutrical? If so, that's good. If not, get Nutrical.
Free feed. Leave food available at all times. Some people say that giving Nutrical regularly might have the opposite effect it is intended for and to just use it as a "rescue" in times of high energy activities. This is a good rule of thumb, if your pup just spent a lot of energy playing and rough housing, give him a little Nutrical during or at the end of the play session.
If he ever seems to not pull out of a lathargic state, get to the vet ASAP. He will need immediate care (ER vet if at night). Don't question it, just do it. They don't have a lot of time when they are this small.
But, if you keep that food out always and give Nutrical when needed, your pup should be fine. Get well versed on the symptoms of hypoglycemia ok?
For food, it takes a while to find what they like. Some are picky eaters. Make sure you are giving puppy food and not dog food. They need those extra nutrients and calories. I highly recommend grain free because so many Yorkies seem to have allergies to grains. I have always fed Blue Buffalo. Others feed Royal Canin, Fromm, and some others that I can't think of. Just try some until you "hit" on one that works. But, I would definitely just leave the bowl with food accessible at all times by your pup.
Hope this has helped some and others will be along with more experienced words of wisdom.
ETA: As for weight, it really varies from pup to pup. I didn't get Mandie until she was a little over 4 months and she weighed 3 lbs 10 oz. I'm not sure of the average weight range for a 3 month old. Again, others will be able to answer that further.