It sounds like your doing things right. You're putting him in his kennel when you can't keep an eye on him and taking him out when he asks.
A couple of things I would try are ,firstly, cleaning where he pottied with a cleaner designed to remove pet scent. When dogs go to the bathroom in the house, the lingering smell, even after cleaning, can cause the to remark in the same spot. I would also consider moving that table temporarily. That should shake things up a little and break his routine of going under it to go to the bathroom. It's not permanent, just till you get the issue straightened out.
Next, I would consider getting him an X Pen. Basically, it's an area significantly bigger than a kennel, but still keeps him contained. It's made of light wire, so it would be easy to put away if company comes over or move from room to room as needed. I know it was probably out there in multiple places, but I first saw it on a Leerburg Kennels dvd. They use it to contain even their adult dogs, who could easily knock it over. They're just used to it. When they have younger dogs it's easy to line the bottom with puppy pads. They also put toys in there and rotate them out once a week.
Lastly, consider that your dog is only 3 months old. It doesn't have full control of it's bladder and won't for some time. I think the rule is that they can be expected to hold it for 1hr per month old they are, not to exceed 9hrs. Of course, you'll have exceptions. You'll find that a younger dog may hold it for longer or shorter time. You should just expect accidents no matter what you do. However, if you keep up the training and stick with it, I'm confident the dog will become a well housebroken pet.
Maybe trying taking him out more frequently whether he asks or not. There may be something outside that triggers him to potty. Other things that encourage using the bathroom in dogs are waking up, eating, drinking, and playing. Make sure you take him out after these activities to avoid some of the accidents. Good luck.