Completely agree with Ghilly on this one. I wouldn't worry at this age either. Check and see if it may be descending and then going back up. Best time to check is after they have been up for a while.
In cases of a veterinary emergency I wholeheartedly agree with you, marksme. However an undescended testicle in a three month old puppy is most definitely not a veterinary emergency, therefore I don't think that lovably17 was negligent or remiss at all by posting their question here to get opinions on what people may think. There ARE, however, many who DO post here who, instead of sitting at a computer typing that their dog hasn't eaten in a week, what do we think could be wrong with it, SHOULD be at the vet (or should have been at the vet four days prior!) instead of sitting at the computer asking questions of a group of non-veterinary people. Those are the ones that I just don't understand!
Ghilly
I would always take the dog to a Vet and get expert advice, instead of trying to find out on an internet site. A good Vet is always your best bet for your dogs health and well being.
If your puppy gets to be 10 or 11 months and there is still no sign of the second testicle, I would make an appointment with the vet to see what's what. Retained testicles are problematic for several reasons. If you are planning to show and/or breed the dog, it cannot happen if he has a retained testicle. Well, showing can't happen. Breeding CAN happen, but it isn't advisable because he will pass along the gene for the retained testicle and that's one thing you don't want nor need in your bloodline. The second (and far more serious) problem is that over time, the retained testicle will wither and become non-functioning in terms of producing sperm, but it can also become cancerous, so if there IS a testicle retained you want to make sure your vet stays on top of the situation and removes it as soon as he figures out it has been up there long enough to be sure it's not coming down, but not long enough to cause a health problem.
The surgery is much more involved than a normal neuter, and it is helpful to do an ultrasound to see exactly where the retained testicle is so that it cuts down on the amount of searching that has to be done once the vet cuts into the abdominal cavity. The recuperation period is more along the lines of a spay than that of a neuter since abdominal surgery is performed, so you'll have to keep him quiet for a while afterward.
Ghilly