A 3 months old puppy is about 8-10 lbs, and the estimate neck size is 13-14 inches. As he/she will get bigger, a 14-16 inches collar will soon be needed. I recommend you to only get the collar once you have your pup in your hands as you can easily buy a wrong size. The best is if you buy an adjustable harness, as it is much better for a Frenchie. Here you can find posts of other Tomkings Frenchie Family members about accessories and harness: Let me see